Americans on Friday began somber ceremonies to mark the 10th anniversary of the 9/11 attacks under the shadow of a new terror threat, as armed police patrolled New York still scarred by Al-Qaeda's assault.

US officials have revealed what Homeland Security called 'a specific, credible but unconfirmed threat' against either New York - where memories of the September 11, 2001 attacks remain fresh - or Washington.

The details remain sketchy, but Vice President Joe Biden said Friday a possible car bomb may be at the heart of any threat, which came just three days before Sunday's poignant remembrances for almost 3,000 people killed in the deadliest attack ever on American soil.

"We don't have a smoking gun, but we do have talk about using a car bomb," Biden told the ABC.

The vice president said the threat - on which both he and President Barack Obama were briefed - contained 'specifics' that made it credible, but 'there is no certitude'.

As New York gears for Sunday's poignant ceremony at Ground Zero, the site where the felled World Trade Center once stood, heavily armed police patrolled the busy metropolis and bomb-sniffing dogs scoured the subway.

US media reported the suspected plot was hatched overseas by Al-Qaeda, perhaps to avenge the killing of Osama bin Laden in a US raid in May.

Few details have been given, but one US official told AFP a car bomb was 'at the top of what we would be looking for'.

ABC News cited intelligence officials as saying the potential plot involved three individuals who entered the country by air last month with the intention of carrying out a vehicle-borne attack on or around the 9/11 anniversary.

It said the plot was ordered by Al-Qaeda's new leader Ayman al-Zawahiri, who has vowed to avenge the killing of bin Laden.

Asked if extremists were plotting to enter the United States with a car bomb, Biden said 'we were told that that was an intention, to get people into the United States to do that'.

Although there was no immediate change to the official US national threat level, New York authorities swiftly ordered sweeping heightened security.

"Obviously we don't want to close the city down ... but you will see vehicle checkpoints in several locations, mostly in Manhattan, but not only in Manhattan," police commissioner Ray Kelly told NY1 television.

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 late on Thursday.

Kelly said late on Thursday the measures included police 'trained in heavy weapons positioned outside of Manhattan to respond citywide'.

Extra shift hours would boost the numbers of patrols around New York, with checks on ferries, tunnels, bridges and landmarks, as well as bomb sweeps in car parks and bag checks on the subway, Kelly said.

Obama and his predecessor George W. Bush, who was president on the day that Al-Qaeda hijackers brought terror from the skies, are due to attend Sunday's ceremonies, along with large crowds.

At Ground Zero, a somber litany of the names of the dead will be read against a backdrop of the progress in rebuilding the World Trade Center, which collapsed in a fiery, toxic heap after being struck by two hijacked airliners.

Sunday will also see the opening of a national 9/11 memorial, consisting of two huge square fountains dug into the footprints of the old Twin Towers.

But events were already getting underway on Friday, with US Secretary of State Hillary Clinton to give a speech after ringing the opening bell at Wall Street.

Defense Secretary Leon Panetta was also addressing emergency workers at the Pentagon, which was also hit by a hijacked plane, before taking part on Friday evening in a special 'Concert to Honour' in Washington.

A fourth plane also crashed in a Pennsylvania field. Despite frequent threats and a string of failed plots, Al-Qaeda has not succeeded in mounting a major attack on US soil since 2001.

However, US officials have warned of a possible backlash following the killing of bin Laden in his secret compound in Abbottabad, Pakistan in May.

US officials have said documents and computer files were seized at the compound showing bin Laden was considering strikes to coincide with the 9/11 anniversary.