Barack Obama has paid tribute to all victims of the 9/11 terrorist attack with the families of those who died when al-Qaida terrorists crashed an aircraft into the Pentagon within an hour of the twin assaults on the World Trade Centre in New York.

"Our hearts still ache for the futures snatched away, the lives that might have been," Obama told family members of the 184 people who died at the Pentagon.

The shadow of possible military action in Syria loomed over the event as Obama vowed to "defend our nation".

"Let us have the wisdom to know that while force is sometimes necessary, force alone cannot build the world we seek," Obama said, after placing a wreath on the Pentagon 9/11 memorial and speaking to the family members of the victims.

He also mentioned the four Americans, including US ambassador Chris Stevens, who were killed in an Islamist militant attack on a diplomatic compound in Benghazi, Libya, on 11 September, 2012.

"We pray for all those who've stepped forward in those years of war, diplomats who serve in dangerous posts, as we saw this day last year in Benghazi," he said.

First Lady Michelle Obama, vice-president Joe Biden and his wife Jill also took part in the ceremony.

At 8.46 am EDT precisely, hundreds of mourners gathered at Ground Zero, the site of the fallen World Trade Centre in New York, to mark in silence the moment when American Airlines flight 11 crashed into the North Tower.

That attack was followed less than 20 minutes later by a second aircraft, United Airlines flight 175, flown into the side of the South Tower.

Both buildings collapsed and 1,753 people died, including 403 police officers and firefighters.

A fourth hijacked flight was brought down in a field in Pennsylvania when passengers of United Airlines flight 93 overwhelmed the terrorists who were believed to be targeting the Capitol building in Washington DC.

The anniversary ceremony included the recitation of the names of the dead while bagpipes and bells rang in the background. No public officials spoke at the ceremony, although former New York mayor Rudolph Giuliani, his successor, Michael Bloomberg, police commissioner Raymond Kelly, and other city and state leaders attended.

Nineteen hijackers died in the four attacks, later claimed by Osama Bin Laden and al-Qaida, which led directly to the US war in Afghanistan and indirectly to the invasion of Iraq.

9/11 counter-demonstration

A 9/11 counter-demonstration of two million bikers who had planned to ride through Washington DC has been denied a permit, according to the Washington Times.

The aim was to protest against a march organised by the American Muslim Political Action Committee, a fringe group that promulgates anti-Semitic conspiracy theories about the 9/11 attacks.

The bikers planned their counter-demonstration to "remember the victims of 9/11 and to honour our armed forces that fought those who precipitated this attack".

"We did the right thing and went through the proper channels to secure a no-stop permit to ride through your great city," the bikers said.

"We wanted to ride an established route, which would have taken us past the Vietnam Memorial to the Lincoln Memorial, across the bridge into Virginia, and that's it. We would have been completely out of Washington DC, and your city would have been back to normal."