Security has been stepped up in New York and Washington as US authorities investigate a 'credible' but unconfirmed terrorist threat ahead of the tenth anniversary of the 11 September attacks.

US citizens have been urged to be on alert as police believe militants may be preparing to carry out a series of car bombings in the two cities.

US Secretary of State Hillary Clinton said it had been decided to reveal the new threat in an attempt to thwart any plot.

"Making it public ... is intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and the ears of vigilance," she said.

It would seem from the threat report "that al-Qaeda, again, is seeking to harm Americans and, in particular, to target New York and Washington," Mrs Clinton added.

Former national security advisor Frances Townsend told CNN that US spy networks had been alerted to a new threat after intercepting communications from a known, reliable operative in Pakistan.

"It's Washington or New York. A car bomb, three men. We know that one or two are US citizens," she said, when asked about the specifics of the threat.

Homeland Security Secretary Janet Napolitano also called on all Americans to remain vigilant and report anything suspicious, vowing to "protect the American people from an evolving threat picture both in the coming days and beyond".

President Barack Obama said on Saturday the United States was stronger 10 years after the 11 September attacks and Americans would "carry on" despite continued threats against their safety.

Marking Sunday's 10th anniversary of the "9/11" attacks on New York, Washington and Pennsylvania, Mr Obama noted that al-Qaeda's strength had been sapped by relentless US efforts in the decade since the tragedy killed nearly 3,000 people.

President Obama in his weekly radio and internet address said "thanks to the tireless efforts of our military personnel and our intelligence, law enforcement and homeland security professionals, there should be no doubt: today, America is stronger and al-Qaeda is on the path to defeat."

Barack Obama will travel to all three sites tomorrow where hijackers turned planes into missiles, bringing down the twin towers of the World Trade Center in New York, hitting the Pentagon in Virginia and crashing into a Pennsylvania field.

Former president George W Bush lay a wreath at the Pentagon in a private ceremony today and is also due to attend tomorrow's ceremonies.

George W Bush also attended a memorial in Shanksville, Pennsylvania, where former president Bill Clinton and current Vice President Joe Biden unveiled a memorial to those who died aboard hijacked United Airlines Flight 93.

Mr Bush honored as heroes the 40 passengers and crew who helped avert further disaster by overpowering the hijackers intent on flying the airliner into the US Capitol, saying they launched the "first counteroffensive in the war on terror."

But the president who launched that war also spoke of the wrenching pain that relatives still feel, 10 years on.

Sombre ceremonies began in the US yesterday to mark the anniversary as armed police patrolled New York's streets and subways.

Trucks and cars were stopped and inspected at vehicle checkpoints and bomb-sniffing dogs searched the subway.

Memories remain strong of the day when al-Qaeda hijackers crashed three passenger planes into the World Trade Center and the Pentagon.

A fourth plane crashed in a Pennsylvania field when the passengers overpowered the hijackers. Almost 3,000 people were killed that day in the worst ever attacks on American soil.