US officials have received "credible but unconfirmed" intelligence that a terrorist attack is being planned for New York or Washington on the anniversary of 9/11.
Associated Press reported a counter-terrorism official as saying the intelligence had been under investigation since it was received late on Wednesday night.
ABC news said intelligence agencies received information that three people had entered the US with the intention of launching a "vehicle-borne" attack on the anniversary of September 11.
The mayor of New York, Michael Bloomberg, announced a press conference for 9.30pm ET (2.30am BST) with the city's police commissioner, Ray Kelly, and a senior FBI official.
In a report on its website, ABC said officials believed the suspected attackers began their journey in Afghanistan, and may have passed through Iran.
AP said officials would not say specifically what is being targeted in New York or Washington, nor the timing of a potential attack. A government official, speaking on condition of anonymity, said the terror alert had not been raised because of the threat.
AP said that, according to its source, the threat came in a single piece of information.
A White House official said President Barack Obama was briefed on Thursday morning.
Matt Chandler, a spokesman for the homeland security department, said in a statement: "As we know from the intelligence gathered from the Osama bin Laden raid, al-Qaida has shown an interest in important dates and anniversaries, such as 9/11. In this instance, it's accurate that there is specific, credible but unconfirmed threat information.
"As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days.
"Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots under way. Regardless, we take all threat reporting seriously, and we have taken, and will continue to take all steps necessary to mitigate any threats that arise.
'We continue to ask the American people to remain vigilant as we head into the weekend."
The homeland security secretary, Janet Napolitano, told reporters last Thursday there was "a lot of chatter" around the anniversary of the attacks but there was no information about a specific threat.
Security measures around the US, including in New York and Washington, have been enhanced in the weeks leading to the anniversary on Sunday.
