NEW YORK (AFP) - The United States marked six years since the September 11 attacks Tuesday with solemn ceremonies but still haunted by Al-Qaeda chief Osama bin Laden, who used the anniversary to praise the hijackers.

In an overcast New York, families of the 2,749 people killed when two planes plowed into the World Trade Center paid their respects near the site as rescue workers read the names of the dead, in what has now become an annual ritual.

With heads bowed, holding photographs of the dead and fighting to hold back the tears, relatives listened as the grim roll call was read out to the haunting strains of a solo cello, flute and guitar.

"We come together again as New Yorkers and as Americans to share a loss that can't be measured and to remember the names of those who can't be replaced," said New York Mayor Michael Bloomberg, introducing the commemorations.

The day of the attacks six years ago was "a day that tore across our history and our hearts," he said.

"We love you and we miss you," said one woman, mourning the loss of her brother. "You're still the best, Salvatore," added another, paying tribute to his fallen firefighter brother.

The ceremony was more muted than in past years. Last year, President George W. Bush visited New York to lay a wreath at the site, but this year attended a private service and observed a moment of silence in Washington.

As in previous years, Bin Laden used the anniversary to release two videotapes, mocking the United States, threatening to escalate the increasingly unpopular war in Iraq and praising hijacker Walid al-Shehri as a "champion."

Shehri was on American Airlines Flight 11, the first jet to crash into the World Trade Center in New York.

The hijacker was "a young man who personally penetrated the most extreme degrees of danger and is a rarity among men: one of the 19 champions," a US-based monitoring group that obtained the video quoted Bin Laden as saying.

The militant Islamist leader remains at large and is believed to be hiding in the mountainous region straddling the Afghanistan-Pakistan border.

For the first time, most of the commemorations in New York were being held at a park near Ground Zero, the area where the Twin Towers once stood, and not on the site where a memorial and other new buildings are being constructed.

The reading of the names was to pause for four moments of silence to mark the exact times that the planes hit the towers and when the massive buildings collapsed into piles of rubble and dust.

Relatives of those killed were then able to descend a long ramp into the World Trade Center site to lay flowers and pause momentarily. The decision not to hold the ceremony at Ground Zero itself has stirred controversy.

But Bloomberg justified the decision on Tuesday, saying people needed to accept the change.

"The place where we used to hold this ceremony is now a construction site. This is probably the last year people will be able to walk down the ramp into the pit," Bloomberg told CNN ahead of the ceremony.

His predecessor, Republican presidential hopeful Rudolph Giuliani, was also due to give a reading, sparking criticism from some of the families of those killed, given his presidential ambitions.

Giuliani has made much of his role as mayor in the aftermath of the attacks, but firefighters especially have criticized the city's response to the disaster and have accused Giuliani of making political capital out of the attacks.

Democratic presidential front-runner Hillary Clinton, a New York senator, is also due to take part in the ceremony, reading the names of the dead.

In the evening, a "Tribute in Light" is to project two massive beams of light into the night sky above Ground Zero to symbolize the collapsed towers.

In Shanksville, Pennsylvania, where hijackers brought down United Airlines Flight 93 in a field after a passenger uprising, tributes were to be held to honor the 40 passengers and crew killed there.

Defense Secretary Robert Gates meanwhile led a ceremony in Washington for the 184 people killed when American Airlines Flight 77 flew into the Pentagon.

The Defense Department at the weekend honored the dead and showed support for US troops, more than 4,100 of whom have been killed in Iraq and Afghanistan since Bush declared a "war on terror" in response to the attacks.

The president's popularity has plummeted since he stood in the ruins of the World Trade Center with a bullhorn six years ago to rally the American people.

In a poignant reminder of the reality of the post-September 11 age, Turkish police defused a powerful bomb hidden under a bus in Ankara, while security was tightened at a US military base in Germany in response to a bomb threat.