United States Secretary of State Hillary Clinton says al Qaeda is behind a specific, credible but unconfirmed report of a threat to harm Americans, notably in New York and Washington.
Sunday is the 10th anniversary of the 9/11 terror attacks on the United States. Nearly 3000 people died in New York, Washington and Pennsylvania on 11 September 2001.
''Al-Qaeda again is seeking to harm Americans and in particular to target New York and Washington,'' Mrs Clinton said on Friday.
''This should not surprise any of us. It is a continuing reminder of the stakes in our struggle against violent extremism, no matter who propagates it, no matter where it comes from, no matter who its targets might be.''
Mrs Clinton says everything is being done to protect people. Police have increased patrols at possible target sites and have been searching vehicles, causing major traffic jams.
"We are taking this threat seriously; federal, state and local authorities are taking all steps to address it."
Mrs Clinton said the decision to go public with the threat was ''intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and ears of vigilance''.
She said documents seized during the raid of Osama bin Laden's Pakistani compound on 1 May showed al-Qaeda was considering strikes to coincide with the anniversary.
Events commemorating the anniversary of the attacks are scheduled throughout the weekend.
President Barack Obama will visit the World Trade Center site, the Pentagon and attend a memorial ceremony in Shanksville, Pennsylvania, where the fourth hijacked plane, United 93, came down.
The BBC reports he will be joined in New York by former President George W Bush.
More checks
New York Mayor Michael Bloomberg said on Thursday night 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.
''There is no doubt that New York is safer now than it was 10 years ago, but there are no guarantees,'' he said. ''We are doing everything that I believe we can do to protect the city, but it is a dangerous world."
Mr Bloomberg said that New York police would be ''deploying additional resources ... some of which you will notice and some of which you will not''.
Reports suggest that intelligence gathered in Pakistan points to a possible car or truck bomb attack against New York or Washington.
ABC TV reported on Thursday evening that three individuals entered the United States in August aiming to carry out a terror attack. It was unclear whether this information was linked to the threat detailed by US officials.
