New York is deploying extra police at key locations including tunnels and bridges in response to intelligence reports of a possible threat linked to the 10th anniversary of the 9/11 attacks, officials said on Thursday.
The police department "is deploying additional resources. Some of which you will notice and some of which you will not," Mayor Michael Bloomberg told reporters, stressing that there was no confirmation of the reported threat.
The city's police commissioner, Raymond Kelly, told the press conference that new measures included police "trained in heavy weapons positioned outside of Manhattan to respond citywide."
Extra shift hours would effectively increase by a third the size of patrols around New York, with checks on ferries, tunnels, bridges and landmarks, Kelly said.
There will be increased towing of illegally parked cars and more bomb detection sweeps in carparks, as well as "increasing the number of bag inspections on the subway," Kelly said.
In addition, "the public is likely to see and may be somewhat inconvenienced by vehicle checkpoints at various locations."
"The threat at this moment has not been corroborated. I want to stress that," Bloomberg said. "It is credible but it has not been corroborated. But we do live in a world where we take these threats seriously."
The possible threat was first announced by federal officials in Washington, who did not confirm the potential locations of an attack.
The scare came days ahead of Sunday's anniversary ceremonies for the September 11, 2001, attacks. President Barack Obama and his predecessor George W. Bush are due in New York on the day.
Officials have also said they believe that Al Qaeda leader Osama bin Laden had been plotting attacks around the anniversary before he was shot dead by US commandos in May.