War-weary Americans yesterday marked the 12th anniversary of the September 11, 2001 attacks, as President Barack Obama - for now holding fire on Syria - said force alone cannot “build the world we seek.”

In New York, Washington and a field in Shanksville, Pennsylvania, relatives of the nearly 3,000 people killed in the plane attacks gathered for sombre remembrance ceremonies.

“Our hearts still ache for the futures snatched away, the lives that might have been,” Obama said at the Pentagon, where 184 died when a hijacked airliner struck the building.

“They left this Earth. They slipped from our grasp.”

US troops invaded Afghanistan shortly after the 9/11 attacks to oust the Taliban for giving refuge to Al Qaeda, and Obama gave thanks to the American forces who served there - but noted the war was now coming to a close.

He said the US would remain vigilant in the face of future terror threats but said military might alone could not bring peace and security.

“Let us have the wisdom to know that while force is at times necessary, force alone cannot build the world we seek,” he said.

On Tuesday, Obama delivered an address to the nation in which he explained why he was for now holding off on punitive military strikes against Syria over its alleged chemical weapons use.

“I will not put American boots on the ground in Syria. I will not pursue an open-ended action like Iraq or Afghanistan,” the president pledged.

Polls show a majority of Americans - weary of war after US interventions in Iraq and Afghanistan - oppose US action in Syria.

In New York, 9/11-era mayor Rudolph Giuliani wiped away a tear at the Ground Zero event, which began with a moment of silence at 8:46am (1246 GMT), marking the time when the first plane smashed into the World Trade Center.

Moments of silence were observed at 9:03am - the time when the second tower was hit; 9:37am, the time of the Pentagon attack, and 10:03am, when a fourth airliner crashed in Pennsylvania after passengers intervened.

Relatives gathered at the 9/11 Memorial around the two huge black fountains that mark the footprints of the Twin Towers - which collapsed in the attacks - for the ritual reading of the names of the dead.  New York Mayor Michael Bloomberg and state Governor Andrew Cuomo attended the ceremony, as did New Jersey Governor Chris Christie.

At the Pentagon, Obama remembered US soldiers, diplomats and intelligence agents who had died since 2001, and said the long ground war in Afghanistan sparked by the attacks was entering its final chapter.