New York and Washington were on high alert last night amid fears Ayman al-Zawahiri, the new al-Qa'ida chief, may be seeking to avenge Osama bin Laden's death by launching a terrorist attack around the anniversary of September 11.
Joe Biden, the US vice-president, said intelligence had "specific" information from a "credible source" that a plot involving "a car bomb" may be under way.
However, he stressed: "We don't have a smoking gun."
Hillary Clinton, the secretary of state, named al-Qa'ida as being behind the suspected threat. It came amid revelations that Britain's MI5 has been advising New York authorities on security.
It is understood that Security Service officials have given briefings on their experience of dealing with IRA vehicle bombs in London.
Both cities were increasing security ahead of ceremonies in commemoration of the 10th anniversary of the attacks, which killed almost 3,000 people.
US President Barack Obama and former president George W Bush are to attend a ceremony at Ground Zero in Manhattan tomorrow.
Police checkpoints were in place on Manhattan streets. Officers were increasing numbers on New York's transport network, making random checks on vehicles and bags, and monitoring airports, bridges and tunnels.
New York Mayor Michael Bloomberg said: "The NYPD, FBI and entire intelligence community have been on heightened alert."
It is thought that three people who entered the US by air last month, one of them a US citizen, are urgently being sought in relation to the suspected threat. However, their identities were not thought to be known last night, and officials continued to search arrivals lists.
Notes seized during the May raid on Bin Laden's compound in Pakistan, in which he was shot dead, raised the possibility of a 9/11 anniversary attack, officials said.
Janice Fedarcyk, the head of the FBI's New York unit, said: "Al-Qa'ida has shown an interest in important dates and anniversaries."
CIA officials reportedly believe those involved embarked on a journey to the US from the tribal areas of Pakistan, where several important al-Qa'ida and Taliban operatives, including al-Zawahiri, are thought to be based.
Multiple reports said officials had "reason to believe" al-Zawahiri was involved.
In a video message released three weeks ago, al-Zawahiri told followers: "America today is staggering. Hunt her down wherever you may encounter her. Hunt her down to cut what is left of her corruption's tail."
Faisal Shahzad, the 32-year-old Pakistani-American who tried to detonate a car bomb in New York's Times Square last year, is believed to have received training in the tribal areas.
Eric Holder, the US attorney general, said the Pakistani Taliban was "intimately involved" in his plot.
Ms Clinton said the unconfirmed report suggested "al-Qa'ida again is seeking to harm Americans".
She promised a "relentless" campaign against it and said a "Global Counter-Terrorism Forum", bringing together international officials, would be established later this month.
Bill Baroni, the deputy head of the Port Authority of New York and New Jersey, which owns the site, said British intelligence officers had been helping to devise its security. Officials were "learning from the experience that London has had".
Ceremonies are to be held tomorrow at the Pentagon in Virginia, where 184 people died when American Airlines flight 77 struck; and in Shanksville, Pennsylvania, where all 40 passengers and crew members died when United Airlines flight 93 was flown into a field.
A ceremony to commemorate the 343 New York firefighters killed in the attacks is due to be held today in New York City. (© Daily Telegraph, London)