A possible al-Quaeda terrorist attack in New York or Washington around the 10th anniversary of September 11th is being investigated by US officials.

The U.S. Department of Homeland Security called the 9/11 anniversary terrorist threat “specific, credible but unconfirmed”, spokesman Matt Chandler said in a statement.

Details were not disclosed, but the official said a possible vehicle-borne attack, perhaps on a transportation hub or bottleneck, could be planned for either New York or Washington, to coincide with the 10th anniversary of 9/11.

A small al-Qaeda-sponsored group is thought to have entered the US in recent days as part of the attack plot. Officials said the individuals originated from the tribal areas of Pakistan along the Afghan border, but one may be a US citizen.

President Barack Obama has been briefed on the threat and has instructed counter-terrorism officials to redouble their efforts.

Security is already tight ahead of the anniversary of the September 11th 2001 attacks New York’s World Trade Center and the Pentagon outside Washington, D.C.

New York mayor Michael Bloomberg said: “Terrorists view the anniversary as an opportunity to strike again. In the next few days we should all keep our eyes wide open.”

After Osama bin Laden was killed, Attorney General Eric Holder said in May that intelligence retrieved from his compound in Pakistan suggested that the al Qaeda mastermind wanted to carry out an attack around the 10th anniversary of 9/11.

"As we know from the intelligence gathered from the [bin Laden] raid,

al-Qaeda has shown an interest in important dates and anniversaries,

such as 9/11,’’ said Chandler.