Iraq's Prime Minister says the country has received credible intelligence that the so-called Islamic State militants plan to attack subway systems in Pairs and the United States.
Prime Minister Haider al-Abadi says he received the information from militants captured in Iraq and concluded it is credible after asking for further details.
Abadi says he has already requested more credible information including names, specific cities, and dates.
Two senior U.S. security officials say the U.S. has no information to support the threat.
But the New York City police department says it is aware of Abadi's warning and in close contact with the FBI to assess the threat.
France has said it will increase security on transport and in public places after a French tourist was killed in Algeria.
Meanwhile, British police have arrested nine men as part of an operation into Islamist-related militancy.
But the police say the arrests are not in response to any immediate threat.
Andrew Dismore is a London Assembly member.
"I first became involved in looking at the activities of extremists in early 1998 when I was an MP, and I particularly started enquiring into the activities of Bakri Muhammad, Abu Qatada, Abu Hamza, and Anjam Choudary's name started to come up very soon after that, when I was investigating what they were up to in terms of radical incitement and extremist views."
Meanwhile, British Foreign Secretary Philip Hammond says Britain could join American and French airstrikes on IS.
The U.S and France have both launched air strikes against IS targets in Iraq.
This week the U.S and five allied Arab states have expanded the aerial campaign in Syria.
