The United States is on high alert following "credible" reports of a possible terror attack ahead of the 9/11 anniversary on Sunday.
US Secretary of State Hillary Clinton has said the administration was "taking seriously" the prospect that al-Qaeda is behind the threat.
Speaking in New York on Friday she said there was "a specific, credible, but unconfirmed report that al-Qaeda, again, is seeking to harm Americans and, in particular, to target New York and Washington."
"This should not surprise any of us, it is a continuing reminder of the stakes in our struggle against violent extremism, no matter who propagates it, no matter where it comes from," she said. "We are taking this threat seriously."
Clinton said the threat was made public on Thursday in order to enlist the "millions and millions of New Yorkers and Americans to be the eyes and ears of vigilance".
Security on public transport in the city has been beefed up, while bridges and tunnels are being watched by the police.
New York mayor Michael Bloomberg said terrorists viewed the anniversary as an opportunity to strike again.
"The NYPD is deploying additional resources around the city ... but there is no reason for the rest of us to change anything in our daily routines," he said.
He also told New York residents: "If you see something, say something."
According to ABC News, the threat came to light following new information uncovered by the CIA indicating that "three individuals entered the US in August by air with the intent to launch a vehicle-borne attack against Washington, DC or New York around the anniversary of 9/11"
President Obama and George W Bush are due to attend an anniversary service, along with Bloomberg and former mayor Rudi Giuliani, at ground zero on Sunday.