The US has warned of a "specific, credible¹ threat" ahead of the 10th anniversary of the 9/11 attacks, possibly in New York or Washington DC.

911事件发生十周年之际，美国政府警告到，可能在纽约或华盛顿将发生一场“特别的、可信的威胁”。

New York Mayor Michael Bloomberg said the threat was "uncorroborated（未经证实的） ", but security would be boosted at bridges, tunnels and on public transport.

Police Commissioner² Raymond Kelly said spot checks in the New York subway would be more frequent in coming days.

President Obama called on the US to "redouble" anti-terror efforts.

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. There was no confirmation³ about whether this information was linked to the threat detailed⁴ 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.

US President Barack Obama and former President George W Bush will attend the ceremony.

White House officials said the president, who spoke⁵ 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 Special Agent in Charge of the New York office, Janet 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 anonymous⁶（匿名的） counter-terrorism official told the Associated Press that authorities had been investigating the intelligence since it was received late on Wednesday.

Mayor Bloomberg told reporters the New York Police Department would be "deploying⁷ 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.

In the days leading up to the anniversary Commissioner 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 Bin⁸ Laden's materials that there was discussion about the 10 year anniversary, the 10 year memorial.

"There is no doubt that New York is safer now than it was 10 years ago, but there are no guarantees. We don't know what we don't know and we are doing everything that I believe we can do to protect the city, but it is a dangerous world," he added.

Officials noted⁹ 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.