US authorities says they have no knowledge of a supposed plot by Islamic State radicals to attack subway systems in the United States and Paris, following reported comments by Iraq's leader about such a threat.
New York police have nevertheless stepped up security at subway stations across the city, but have stressed they have no specific information of an imminent threat.
Iraqi Prime Minister Haidar al-Abadi was earlier quoted as saying at the United Nations that the alleged plot had been revealed following a series of arrests in Iraq.
"We've seen the reports of Prime Minister Abadi's comments," said National Security Council spokeswoman Caitlin Hayden.
"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."
People briefed on the matter told AFP that US authorities did not believe there was a "current" or "specific" threat to subway systems.
A senior US official, meanwhile, said no one in the US government was aware of such a plot.
"It was not raised with us in our meetings with Iraqi officials here in New York," said the official.
Despite that, New York Police Chief William Bratton announced heightened security measures in the city.
"We do not have any confirmed threat directed against New York City or its subway system," Bratton said.
"We always err on the side of caution."
Bratton said extra police officers had been placed throughout the city and at the subway's 450 stations.
France, which has joined the US in carrying out air raids on Islamic State targets in Iraq, earlier reinforced security in public places and on public transport.
But a government source said that the move was ordered before Abadi's comments out of a more general sense of caution.
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