President Barack Obama and his Republican challenger Mitt Romney suspended their advertising campaigns Tuesday to mark the 11th anniversary of the 9/11 attacks on the United States.

Both sides have engaged in a series of negative television spots as they seek the upper hand in the neck-and-neck race for the White House in the eight weeks until the 6 November election.

Obama had no political events planned for Tuesday, but was due to head to the Pentagon which was hit by a passenger jet in one of the attacks, before going to visit wounded American soldiers at Walter Reed military hospital.

Earlier at the White House, the president and his wife Michelle Obama walked out on to the South Lawn as a bell tolled, then bowed their heads and stood for a moment's silence at 8.46 am, the time the first plane struck the north tower of the World Trade Center in New York.

Romney, who last week launched a fresh, multi-state advertising blitz that hammered Obama over his economic record, was due in Nevada later Tuesday to address the National Guard Association Convention, in Reno.

"Eleven years ago, evil descended upon our country, taking thousands of lives in an unspeakable attack against innocents," Romney said in a statement.

"America will never forget those who perished. America will never stop caring for the loved ones they left behind. And America shall remain ever vigilant against those who would do us harm," he added.

"On this most somber day, those who would attack us should know that we are united, one nation under God, in our determination to stop them and to stand tall for peace and freedom at home and across the world."