WASHINGTON (AFP) - US President Barack Obama on Wednesday mourned the lives lost 12 years ago in the September 11, 2001 attacks, saying "our hearts still ache" for the nearly 3,000 people killed.

Against the backdrop of possible US military action against Syria, Mr Obama marked the 12th anniversary of the 9/11 attacks first at the White House, and then at a solemn Pentagon ceremony attended by families of 9/11 victims and senior officers.

"We pray for the memory of all those taken from us - nearly 3,000 innocent souls," Mr Obama said.

"Our hearts still ache for the futures snatched away, the lives that might have been," he said.

"They left this Earth. They slipped from our grasp."

Quoting the Bible, Mr Obama spoke of "the miracle of restoration", paying tribute to the resilient spirit of victims' families, saying he was "amazed at the will that you've summoned in your lives to lift yourselves up and to carry on."

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

He said the country 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.

Nearly 3,000 people were killed in the 2001 attacks in New York, Washington and Shanksville, Pennsylvania.

At the Pentagon, 184 died when a hijacked airliner struck the building.