At a sombre ceremony, against a backdrop of pouring rain, Barack Obama tried to use the anniversary of 9/11 to rally Americans behind the war in Afghanistan, as he laid a wreath at a memorial to those killed during the attack on the Pentagon.

Eight years after the hijackings, which killed nearly 3000 people, the President said the nation must "renew our resolve against those who perpetrated this barbaric act and plot against us still.

"No turning of the seasons can diminish the pain and the loss of that day," he said. "No passage of time, no dark skies can ever dull the meaning of this moment. In the defence of our nation we will never waver."

The service, the first 9/11 commemoration since President Obama took office, took place a stone's throw from the site where 125 people died when American Airlines flight 77 crashed into the US military headquarters.

Despite falling support for the war, with a record 44 US casualties in July, President Obama recently sent 21,000 more troops to the country, calling it a "war of necessity" and saying the perpetrators of 9/11 planned to kill more Americans.