9/11: US warns of 'credible' anniversary threat
The US has warned of a "specific, credible threat" ahead of the 10th anniversary of the 9/11 attacks, possibly in New York or Washington DC.
New York Mayor Michael Bloomberg said the threat was "uncorroborated", but security would be boosted at bridges, tunnels and on public transport.
Police Commissioner Raymond Kelly said spot checks in the New York subway would be more frequent in coming days.
President Barack Obama called on the US to "redouble" anti-terror efforts.
The authorities say intelligence gathered in Pakistan points to possible car or truck bomb attacks at strategic points in New York or Washington.
US TV network ABC News reports that three individuals - one a US citizen - entered the US in August aiming to carry out a terror attack. It was unclear whether this information was linked to the threat detailed by US officials.
A service marking the 10th anniversary of the terror attacks in which almost 3,000 people died will be held on the site of the World Trade Center on Sunday.
President Barack Obama and former President George W Bush will attend the ceremony.
White House officials said Mr Obama, who spoke to Congress about a jobs plan on Thursday night, was briefed on the threat earlier in the day.
Police chief's concern
Speaking at a news conference alongside Mr Bloomberg and Mr Kelly, the FBI Assistant Director in Charge of the New York office, Janice Fedarcyk, confirmed the threat.
"Al-Qaeda has shown an interest in important dates and anniversaries. In this instance it is accurate that there is credible, specific but unconfirmed information," she said.
An anonymous counter-terrorism official told the Associated Press that authorities had been investigating the intelligence since it was received late on Wednesday.
Mr Bloomberg told reporters that New York police would be "deploying additional resources... some of which you will notice and some of which you will not".
These measures could include checking bags and additional security personnel on public transport.
The UK Foreign Office warned Britons in the US to remain vigilant, warning that an attack could happen "in places frequented by expatriates and foreign travellers".
In the days leading up to the anniversary Mr Kelly told the BBC he had concerns about a new threat.
"We are worried specifically about something happening on the anniversary of 9/11 because we saw on some of Bin Laden's materials that there was discussion about the 10-year anniversary, the 10-year memorial," he said.
"There is no doubt that New York is safer now than it was 10 years ago, but there are no guarantees. We are doing everything that I believe we can do to protect the city, but it is a dangerous world."
Officials said that documents seized during the raid of Osama bin Laden's compound on 2 May 2011 in Abbottabad, Pakistan, showed the al-Qaeda leader was considering strikes to coincide with the anniversary.