Iraq has received "credible" intelligence that the Islamic State of Iraq and Levant (ISIL) group plans to attack subway systems in Paris and in the US, according to Iraq's prime minister.
However, American and French officials say they have no evidence to back up Haider al-Abbadi's claim.
Abbadi's comments were met with surprise and cautious scepticism by security, intelligence and transit officials in both countries.
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He said his government had received the information on Thursday morning from fighters captured in Iraq and concluded it was credible after asking for further details.
The attacks, he said, were plotted from inside Iraq by "networks" of the ISIL, a self-declared jihadist group which controls large areas in Syria and Iraq.
"They plan to have attacks in the metros of Paris and the US," Abbadi 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."
Abbadi did not provide any further details.
'No confirmed threats'
Ben Rhodes, a deputy national security adviser to US President Barack Obama, said the US had "not confirmed any specific threat".
"What we have 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, who was travelling with Obama on Air Force One from New York, said.
"We would certainly take seriously any information they are learning."
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French security services also said they had no information confirming Abbadi's statement, a French government official said, speaking on condition of anonymity.
The Associated Press said that several French officials contacted told the news agency that they were aware of no plot.
The reports came as French fighter jets struck ISIL targets in Iraq while US warplanes hit them in Syria.
The French strikes were an answer to the beheading of a French tourist in Algeria by fighters, who said the killing was punishment for the French decision last week to become the first European country to join the US-led bombing campaign.
In Washington, the Pentagon released cockpit video of guided missiles from F-15 jets striking oil refineries and ISIL compounds in Syria.
The air strikes carried out over Wednesday and Thursday hit at the small-scale crude refineries that ISIL fighters use to generate up to $2m in revenues per day.
The strikes by US and Arab warplanes against the oil refineries killed 14 fighters but also left five civilians dead, including a child, according to the Britain-based monitoring group the Syrian Observatory For Human Rights.
The Pentagon said it was aware of reports of civilian casualties, and was investigating, but insisted raids were carried out with precision.
Masked man 'identified'
Separately, the FBI said it had identified a fighter with a British accent seen executing Western hostages in ISIL videos.
James Comey, FBI director, said officers had identified the masked man in videos with a knife at the beheading of two American hostages in recent weeks.
"I'm not going to tell you who I believe it is," Comey said. He said he knew the person's nationality, but declined to give further details.
Meanwhile, in London, police arrested nine people suspected of links to radical extremism, including an Islamic preacher.
If the British parliament votes to take part in the bombing campaign, the Royal Air Force will join jets from the US, France, Saudi Arabia, the UAE, Bahrain and Jordan hitting ISIL targets.