Authorities in the United States are tightening security ahead of Sunday's tenth anniversary of the 11 September terrorist attacks, following what has been described as a new credible threat.

The US Department of Homeland Security said there was specific, credible but unconfirmed information.

US Secretary of State Hillary Clinton today said that al-Qaeda was behind the threat to harm Americans, notably in New York and Washington.

US media reports suggest militants may have prepared to try to carry out a series of car-bombings in the two areas.

However this information is only from one source and has to be treated with some caution.

New Yorkers are said to be taking it in their stride.

Mayor of New York Michael Bloomberg has urged people to remain vigilant, but said residents should go about their business as usual.

Extra police are being deployed at key locations in New York, including tunnels and bridges.

The police department "is deploying additional resources... some of which you will notice and some of which you will not," Mayor Bloomberg told reporters.

New York City 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, Commissioner Kelly said.

There will be increased towing of illegally parked cars and more bomb detection sweeps in car parks, as well as "increasing the number of bag inspections on the subway," he added.

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," Mr 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.

US 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.