The US is warning of a "credible threat" in advance of the 10th anniversary of the 9/11 attacks, possibly in New York or Washington.
The Department of Homeland Security said in a statement the threat was "specific, credible but unconfirmed."
Separately, ABC News reports that three individuals - one a US citizen - entered the US in August aiming to carry out a terror attack.
President Obama called on the US to "redouble" anti-terror efforts.
White House officials said the president, who spoke to Congress about a jobs plan on Thursday night, was briefed on the threat earlier in the day.
An anonymous counter-terrorism official told the Associated Press that authorities have been investigating the intelligence since it was received late on Wednesday.
Officials noted that documents seized during the raid of Osama bin Laden's Pakistan compound in May showed the al-Qaeda leader was considering strikes to coincide with the anniversary.
http://www.bbc.co.uk/news/world-us-canada-14848962