US officials have no indication of a plot by Islamic State militants to attack underground rail systems in the US and Paris.Iraqi Prime Minister
Haider al-Abadi has said his intelligence officials had uncovered plans for such an attack.
Speaking on the sidelines of the United Nations General Assembly, he said the details he received looked credible.
But senior US sources have said they have no knowledge of a plot on any subway systems.
Mr Abadi said he had passed the information to President Barack Obama.
He added the he was not sure if the attack was imminent but said the information was gleaned from arrests of IS militants in Iraq.
An Iraqi official at the UN has told the BBC that several IS fighters had been captured and told Iraqi intelligence that French and American recruits had "an imminent plot" to hit the metro in Paris, and then hit the United States.
The official did not say where in the US.
US National Security Council spokeswoman Caitlin Hayden said: "We have not confirmed such a plot, and would have to review any information from our Iraqi partners before making further determinations.
"We take any threat seriously and always work to corroborate information we receive from our partners.
"We're obviously very focused on the issue of foreign fighters, as you saw evidenced yesterday at the Security Council session the president chaired."
IS has seized large areas of Syria and Iraq in recent months and Western leaders have expressed concern that it could be hatching plots against the West.
The US launched nearly 200 air strikes against the militants in Iraq since August and expanded the operation against IS to Syria on Monday.
France has also taken part in air strikes against IS in Iraq.