The victims of September 11 will be remembered "no matter how many years pass," President Barack Obama said on Tuesday during one of three main ceremonies marking the 11th anniversary of the attacks in which nearly 3,000 people were killed by airliners hijacked by Islamist militants.

Two of the passenger jets brought down the Twin Towers of New York City's World Trade Center, another hit the Pentagon outside Washington and a fourth crashed in a field in Pennsylvania when passengers aboard that flight fought back against the hijackers.

Obama, speaking at the Pentagon where 184 people were killed, told victims' families that the whole country shares their loss.