NEW YORK, Sept. 25 -- The United States has not confirmed that the Islamic State (IS) militants planned to attack subway system in New York and Paris, and found no evidence to back up Iraqi prime minister's claims.

Captive IS militants told Iraqi intelligence agency of an alleged plot to attack subways in New York and Paris, Iraqi Prime Minister Haider al-Abadi on Thursday told journalists on the sidelines of a gathering of world leaders at the UN General Assembly.

"From the details I have received, it looks credible," he said. "They are not Iraqis. Some of them are French, some of them are Americans. But they are in Iraq." He was not sure whether the attacks are imminent, he said.

Ben Rhodes, a deputy national security adviser to U.S. President Barack Obama, said his country 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 channels," Rhodes told reporters traveling with Obama on Air Force One from New York.

But he said the government would take seriously the information.

French security services also said they had no information confirming al-Abadi's statement, an anonymous official said.

In response to the alert, more police were sent to patrol subways and streets in New York city, which was already on high alert for the UN meeting, Police Commissioner Bill Bratton said.

The United States and France have both launched airstrikes against the IS targets in Iraq amid U.S.-led campaign to destroy the extremist groups. This week, the United States and its Arab allies also launched airstrikes on IS targets in Syria.