A warning by Iraq's prime minister that Islamist militants planned to attack U.S. subways sent political leaders scrambling on Thursday to assure the public it was safe to ride the subways and travel the streets of the nation's largest city.

New York Governor Andrew Cuomo and New York City Mayor Bill de Blasio each had the same idea - take a subway ride to a busy transit hub to announce that everything was under control.

"I have a simple message for all New Yorkers. There is no immediate credible threat to our subway system," de Blasio said at a hastily arranged news conference at Manhattan's Union Square where he arrived by subway from City Hall.

A few minutes earlier, and a few blocks away, the governor staged his appearance by taking a subway to Penn Station, a major rail terminal.

"You are going to see a greater police presence than you have seen before," Cuomo said. "Don't be alarmed. If anything, that should be comforting."

While the newest threat was unsubstantiated, the added police presence was a precaution, said New York Police Commissioner William Bratton, who appeared alongside the mayor.

Their public reassurances were reminiscent of pronouncements by former mayor Rudolph Giuliani when New Yorkers, devastated by the Sept. 11 attacks on the World Trade Center, were fearful of further devastation to the city.

U.S., Paris subways are alleged target

The security concerns arose after Prime Minister Haider al-Abadi said Iraq had received what he termed credible intelligence that ISIS militants planned to attack subway systems in Paris and the United States.

Senior U.S. and French officials promptly said they had no evidence to back up the claim. Abadi made his remarks while in New York for the annual meeting of the UN General Assembly.

New York Mayor Bill de Blasio, second from right, and New York City Police Commissioner William Bratton, right, enter the City Hall subway station while on their way to give a news conference in New York on Thursday. (Adrees Latif/Reuters) Extra police patrolled among crowds of commuters and tourists dragging luggage through busy subway hubs and at Penn Station and the Port Authority Bus Terminal during the evening rush hour.

Uniformed officers, several with explosive-sniffing dogs, were stationed every few hundred metres, with tables set up to conduct random bag searches.

New Yorker Richard Betancourt, 64, gave a thumbs-up to a group of police officers keeping watch at a 42nd Street subway.

"They'd be stupid not to go for it," he said of the visible security presence. "New York is the No. 1 target."

"It's something that these nut jobs might see, and then they'll think twice before they do anything, if they have any brains," he said.

The police commissioner said security would be added throughout the city, with heavily armed teams of counter-terror police, stepped-up searches and co-ordination with private security workers.

Cuomo said authorities already had boosted security in response to potential threats from ISIS and other militant groups. The New York governor, together with his New Jersey counterpart, Chris Christie, announced a bi-state initiative to that end on Wednesday.