Frantic Repairs go on at Fukushima Daiichi Nuclear Power Station plant, Japan

Japanese engineers battled on Friday to cool spent fuel rods and restore electric power to pumps at the stricken Fukushima Daiichi Nuclear Power Station plant as new challenges seemed to accumulate by the hour, with steam billowing from one reactor and damage at another apparently making it difficult to lower temperatures.

As the crisis seemed to deepen, Japan's nuclear safety agency raised the assessment of its severity from 4 to 5 on a 7-level international scale, news reports said. Level 4 is for incidents with local consequences while level denotes broader consequences. It was not immediately clear why the action had been taken. The partial meltdown at Three Mile Island in 1979 was rated 5 and the Chernobyl disaster in 1986 was rated 7.

In the past two days, Japanese officials have focused their efforts on cooling spent fuel rods in a storage pool in Reactor No. 3 but steam, probably carrying radioactive particles, was also seen on Friday rising from Reactor No. 2. which was hit by an explosion on Tuesday.

Additionally, a senior Western nuclear industry executive said that there also appears to be damage to the floor or sides of the spent fuel pool at Reactor No. 4, and that this is making it extremely hard to refill the pool with water. The problem with No. 4 was first reported by The Los Angeles Times.

Engineers had said on Thursday that a rip in the stainless steel lining of the pool at Reactor No. 4 and the concrete base underneath it was possible as a result of earthquake damage. The steel gates at either end of the storage pool are also vulnerable to damage during an earthquake and could leak water if they no longer close tightly. The senior executive, who asked not to be identified because his comments could damage business relationships, said Friday that a leak had not been located but that engineers had concluded that it must exist because water sprayed on the storage pool has been disappearing much more quickly than would be consistent with evaporation.

"They have to figure out what to do, and certainly you can't have No. 2 going haywire or No. 3 going haywire at the same time you're trying to figure out what to do with No. 4," said the executive, who said he had learned of the problem from industry contacts in Japan.

One concern at No. 4 has been a fire that was burning there earlier in the week; American officials are not convinced that the fire has gone out. American officials have also worried that the spent-fuel pool at that reactor has run dry, exposing the rods.

Experts are studying the problem, while technicians continue to combat the risk of overheating at the storage pool at Reactor No. 3 and try to finish connecting a new high-power line from the national grid to Reactor No. 2.

The new setbacks emerged as the first readings from American data-collection flights over plant in northeastern Japan showed that the worst contamination has not spread beyond the 19-mile range of highest concern established by Japanese authorities.

But another day of frantic efforts on Thursday to cool nuclear fuel in the troubled reactors and in the plant's spent-fuel pools resulted in little or no progress, according to United States government officials. The crisis at the plant seemed increasingly to have produced divergent narratives in Washington and Tokyo, with Japanese officials stressing the efforts they were making to tame the damaged plant and American officials highlighting the challenges.

On Friday, water cannons sprayed the stricken Reactor No. 3 on Friday afternoon, live video on the public broadcaster NHK suggested. The footage showed a stream of water aimed at the damaged reactor building, which was rocked by an explosion on Monday, and occasional clouds of steam rising into the air. The Defense Ministry said soldiers of the Japan Self-Defense Force were manning seven trucks that would approach the No. 3 building one after the other, staying near the reactor for only a short period to minimize soldiers' exposure to radiation.

With a first phase of the operation completed, "the water is likely to have reached the target," said Shigeru Iwasaki, the chief of staff of Japan's Air Self Defense Force. The were reported to have returned to the plant later on Friday. Perhaps because of the difficulties experienced on Thursday trying to accurately drop water from helicopters, the military announced on Friday that it was halting those efforts for at least a day. But the limited flow of information about what is happening continued to provoke international concerns.

After a meeting with Prime Minister Naoto Kan on Friday, Yukiya Amano, the head of the International Atomic Energy Agency, said he would dispatch a team "within days" to monitor radiation near the damaged plant.

At the meeting, Mr. Amano, who had just arrived from the agency's headquarters in Vienna, said Mr. Kan agreed on the necessity to disclose as much information as possible on the unfolding nuclear crisis in Fukushima. "What's important is coordination with international society and better transparency," Mr. Amano told reporters before the meeting.

The French government is airlifting 95 tons of boron to Japan, French officials said on Thursday. Boron absorbs neutrons during a nuclear reaction and can be used in an attempt to stop a meltdown if the zirconium cladding on uranium fuel rods is compromised. Tokyo Electric said earlier this week that there was a possibility of "recriticality," in which fission would resume if fuel rods melted and the uranium pellets slumped into a jumble together on the floor of a storage pool or reactor core. Spraying water on the uranium under these conditions can actually accelerate fission, said Robert Albrecht, a nuclear engineer who worked as a consultant to the Japanese nuclear reactor manufacturing industry in the 1980s and visited the Fukushima Daiichi reactor then.

The data from American flights was collected by the Aerial Measuring System, among the most sophisticated devices rushed to Japan by the Obama administration in an effort to help contain a nuclear crisis that a top American nuclear official said Thursday could go on for weeks.

Strapped onto a plane and a helicopter that the United States flew over the site, with Japanese permission, the equipment took measurements that showed harmful radiation in the immediate vicinity of the plant ' a much heavier dose than the trace levels of radioactive particles that make up the atmospheric plume covering a much wider area.

While the findings were reassuring in the short term, the United States declined to back away from its warning to Americans there to stay at least 50 miles from the plant, setting up a far larger perimeter than the Japanese government had established. American officials did not release specific radiation readings.

American officials said their biggest worry was that a frenetic series of efforts by the Japanese military to get water into four of the plant's six reactors ' including the water cannons and helicopters ' showed few signs of working.

"This is something that will likely take some time to work through, possibly weeks, as eventually you remove the majority of the heat from the reactors and then the spent fuel pool," said Gregory Jaczko, the chairman of the United States Nuclear Regulatory Commission, briefing reporters at the White House.

The effort by the Japanese to hook some electric power back up to the plant did not begin until Thursday and even if it succeeds, it was unclear whether the cooling systems, in reactor buildings battered by a tsunami and then torn apart by hydrogen explosions, survived the crisis in good enough shape to be useful.

"What you are seeing are desperate efforts ' just throwing everything at it in hopes something will work," said one American official with long nuclear experience who would not speak for attribution. "Right now this is more prayer than plan."

On Thursday, American President, Obama said that the crisis had convinced him to order the Nuclear Regulatory Commission to conduct a comprehensive review of the safety of nuclear plants in the United States.

After a day in which American and Japanese officials gave radically different assessments of the danger from the nuclear plant, the two governments tried on Thursday to join forces.

Experts met in Tokyo to compare notes. The United States, with Japanese permission, began to put the intelligence-collection aircraft over the site, in hopes of gaining a view for Washington as well as its allies in Tokyo that did not rely on the announcements of officials from the Tokyo Electric Power Company, which operates Fukushima Daiichi.

American officials say they suspect that the company has consistently underestimated the risk and moved too slowly to contain the damage.

Aircraft normally used to monitor North Korea's nuclear weapons activities ' a Global Hawk drone and U-2 spy planes ' were flying missions over the reactor, trying to help the Japanese government map out its response to last week's 9.0-magnitude earthquake, the tsunami that followed and now the nuclear disaster.

Obama made an unscheduled stop at the Japanese Embassy to sign a condolence book, writing, "My heart goes out to the people of Japan during this enormous tragedy." He added, "Because of the strength and wisdom of its people, we know that Japan will recover, and indeed will emerge stronger than ever."

Later, he appeared in the Rose Garden at the White House to offer continued American support for the earthquake and tsunami victims, and technical help at the nuclear site.

But before the recovery can begin, the nuclear plant must be brought under control. American officials, meanwhile, remained fixated on the temperature readings inside Reactor No. 2 and two others that had been operating until the earthquake shut them down, as well as at the plant's spent fuel pools, looking for any signs that their high levels of heat were going down. If the fuel rods are uncovered and exposed to air, they heat up and can burst into flame, spewing radioactive elements. So far the officials saw no signs of dropping temperatures. And the Web site of the International Atomic Energy Agency, the United Nations nuclear watchdog, made it clear that there were no readings at all from some critical areas. Part of the American effort, by satellites and aircraft, is to identify the hot spots, something the Japanese have not been able to do in some cases.

Critical to that effort are the "pods" flown into Japan by the Air Force over the past day. Made for quick assessments of radiation emergencies, the Aerial Measuring System is an instrument system that fits on a helicopter or fixed-wing aircraft to sample air and survey the land below.

Daniel B. Poneman, the deputy secretary of energy, said at a White House briefing on Thursday that preliminary results of the initial flights "are consistent with the recommendations that came down from the chairman of the Nuclear Regulatory Commission," which led to the 50-mile evacuation guideline given to American expatriates. Although the worst contamination is closer to the plant, the recommendation takes into account the possibility of shifting winds or greater emissions.

The State Department has also said it would fly out of the country any dependents of American diplomats or military personnel within the region of the plant and as far south as Tokyo. Space will be made for other Americans who cannot get a flight, it said.

Getting the Japanese to accept the American detection equipment was a delicate diplomatic maneuver, which some Japanese officials originally resisted. But as it became clear that conditions at the plant were spinning out of control, and with Japanese officials admitting they had little hard evidence about whether there was water in the cooling pools or breaches in the reactor containment structures, they began to accept more help.

The sensors on the instrument pod are good at mapping radioactive isotopes, like cesium 137, which has been detected around the nuclear complex and has a half-life of 30 years. In high doses, it can cause acute radiation sickness. Lower doses can alter cellular function, leading to an increased risk of cancer.

Cesium 137 can enter the body through many foods, including milk.

On Wednesday, when the American Embassy in Tokyo, on advice from the Nuclear Regulatory Commission, told Americans to evacuate a radius of "approximately 50 miles" around the Fukushima plant, the recommendation was based on a specific calculation of risk of radioactive fallout in the affected area.

In a statement, the commission said the advice grew out of its assessment that projected radiation doses within the evacuation zone might exceed one rem to the body or five rems to the thyroid gland. That organ is extremely sensitive to iodine 131 ' another of the deadly byproducts of nuclear fuel, this one causing thyroid cancer.

The commission says that the average American is exposed to about 0.62 rem of radiation each year from natural and manmade sources.

The American-provided instruments in Japan measure real levels of radiation on the ground. In contrast, scientists around the world have also begun to draw up forecasts of how the prevailing winds pick up the Japanese radioactive material and carry it over the Pacific in invisible plumes.

Private analysts said the United States was also probably monitoring the reactor crisis with spy satellites that can spot the heat from fires ' helping it independently assess the state of the reactor complex from a distance.