Americans Friday began somber ceremonies to mark the 10th anniversary of the 9/11 attacks as armed police patrolled New York streets and subways to thwart a new Al-Qaeda-linked terror threat.

US Secretary of State Hillary Clinton said there was news "of a specific, credible, but unconfirmed report that Al-Qaeda, again, is seeking to harm Americans and, in particular, to target New York and Washington."

Memories remain raw in New York, and across the country, of the day on September 11, 2001, when Al-Qaeda hijackers slammed three passenger planes into the World Trade Center and the Pentagon, in the nation's capital.

A fourth plane crashed in a Pennsylvania field when the passengers overpowered the hijackers seeking to end the worst ever attacks on American soil.

Clinton was speaking after a minute's silence Friday at the New York Stock Exchange to remember almost 3,000 people killed that day -- many of them staff working in the Twin Towers which collapsed in a fiery heap after the attacks.

Vice President Joe Biden said a possible car bomb may be at the heart of the latest scare which comes ahead of Sunday's poignant remembrances for the 10th anniversary.

"We don't have a smoking gun, but we do have talk about using a car bomb," Biden told ABC television.

President Barack Obama was briefed again Friday on the threat and repeated his order for security agencies to "redouble" efforts to take all necessary precautions, his spokesman Jay Carney said.

But there have been no changes to Obama's plans to attend ceremonies on Sunday at Ground Zero in New York, and Shanksville, Pennsylvania where a memorial to the dead is to be unveiled on Saturday.

Administration officials insisted they have weakened Al-Qaeda, especially after the killing of its leader Osama bin Laden in a US raid on his Pakistan compound in May.

"Our nation is not only safer, but stronger than ever before," Attorney General Eric Holder insisted at a ceremony Friday, under strong, steady rainfall in Washington at the National Law Enforcement Officers Memorial.

Holder, the country's top law enforcement official, laid a wreath and led the reading of the names of 72 law enforcement officer who died on 9/11.

In New York, heavily armed police patrolled the busy streets and bomb-sniffing dogs scoured the subway.

Former mayor Rudy Giuliani, who guided New York during the dark days after the attacks, told CNN the new threat was serious.

While America is safer now than before 9/11, the anniversary should serve as a reminder that "these people who came here to kill us on September 11 want to come and kill us over and over again," he added.

America's response to 9/11 showed that when attacked, "we will come and get you," Defense Secretary Leon Panetta warned Friday.

"The people who attacked us on 9/11 were trying to weaken America, trying to hurt America. And instead they strengthened us," Panetta told police and emergency workers. "Because you don't mess with this country."

He was speaking as he thanked the firefighters and other "first responders" who rushed to the Pentagon after it was struck by a hijacked airliner.

US media reported the new suspected plot was hatched overseas by Al-Qaeda, perhaps to avenge bin Laden's death.

ABC News cited intelligence officials as saying the potential plot involved three individuals who entered the country by air last month planning to carry out a vehicle-borne attack around the 9/11 anniversary.

It said the plot was ordered by Al-Qaeda's new leader Ayman al-Zawahiri, who has vowed to avenge bin Laden's death.

Former president George W. Bush, who was the commander-in-chief on the day that the hijackers rained terror from the skies, is also due to attend Sunday's ceremonies, along with large crowds.

At Ground Zero, a somber litany of the names of the dead will be read against a backdrop of the progress in rebuilding the World Trade Center.

Sunday will also see the opening of a national 9/11 memorial, consisting of two huge square fountains dug into the footprints of the old Twin Towers.