Islamic State militants plan to attack subway systems in Paris and the United States

New York - Iraq has "credible" intelligence that Islamic State of Iraq and Syria (ISIS) militants plan to attack subway systems in Paris and the United States, the prime minister said on Thursday, but US and French officials said they had no evidence to back up his claims.

Prime Minister Haider al-Abadi's comments were met with surprise by security, intelligence and transit officials in both countries. New York's leaders scrambled to ride the subway to reassure the public that the nation's largest city was safe.

Abadi said he received the information on Thursday morning from militants captured in Iraq and concluded it was credible after requesting further details. The attacks, he said, were plotted from inside Iraq by "networks" of the ISIS, also known as IS or ISIL.

"They plan to have attacks in the metros of Paris and the US," Abadi told a small group of US reporters while in New York for the annual meeting of the UN General Assembly. "I asked for more credible information. I asked for names. I asked for details, for cities, you know, dates. And from the details I have received, yes, it looks credible."

Some Iraqi officials in Baghdad questioned Abadi's comments. One high-level Iraqi government official told Reuters it appeared to be based on "ancient intelligence". Another called it "an old story". Both spoke on condition of anonymity.

Abadi did not provide further details. A senior Iraqi official traveling with him later said Iraqi intelligence had uncovered "serious threats" and had shared this information with its allies' intelligence agencies.

"A full assessment of the veracity of the intelligence and how far the plans have gone into implementation is ongoing," the official said.

Ben Rhodes, a deputy national security adviser to US President Barack Obama, said the United States had "not confirmed any specific threat".

"What we've consistently said to the Iraqis is if they have information that is relevant to terrorist activity or terrorist plotting, that they can and should share that through our intelligence and law enforcement challenges," Rhodes told reporters traveling with Obama on Air Force One from New York.

"We would certainly take seriously any information they are learning," he said.

French security services also said they had no information confirming Abadi's statement, a French government official said, speaking on condition of anonymity.

Stronger transit security

New York Governor Andrew Cuomo, New York City Mayor Bill de Blasio and other local officials suggested they were unfazed, updating their public schedules on Thursday to add trips on the city's subway system to reassure millions of daily commuters.

"We are convinced New Yorkers are safe," de Blasio said at a press conference at a lower Manhattan subway station as he stood alongside Bill Bratton, the police commissioner.

Bratton, however, said in response to Abadi's comments that he sent more police to patrol subways and streets in the city which was already on high alert because of the UN meeting. Police will also increase security in and around Yankee Stadium in the Bronx, where baseball star Derek Jeter is due to play his last home game on Thursday before retiring at the end of the season.

Officials in Chicago and Washington DC said they knew of no threats to their transit systems.

The United States and France have both launched air strikes against ISIS targets in Iraq as part of a US-led campaign to "degrade and destroy" the radical Sunni militant group, which has seized a third of both Iraq and Syria.

Abadi disclosed the intelligence while making a case for Western and Arab countries to join that campaign. "We want to increase the number of willing countries who would support this," he said. "This is not military. This is intelligence. This is security. The terrorists have a massive international campaign. Don't underestimate it."

In the past, the United States had received threats that various militant groups were targeting transportation systems but there is no recent information about an imminent plan by ISIS, one US official said, speaking on the condition of anonymity.

Abadi also said that Iraq did not want to see foreign "boots on the ground," but stressed the value of providing air cover, saying Iraq's air force did not have sufficient capability.

He said Australia was "very interested" in participating, though he did not provide details. He also voiced optimism about a planned British parliament vote on Friday on the matter, saying "they reckon it will be successful".

Earlier on Thursday, France said it would increase security on transport and in public places after a French tourist was killed in Algeria, and said it was ready to support all states that requested its help to fight terror.