Americans on Wednesday marked the 12th anniversary of the September 11, 2001 attacks in somber fashion, with ceremonies of remembrance in New York and Washington.

President Barack Obama, Vice President Joe Biden and their wives observed a moment of silence on the White House lawn. The president was to deliver remarks at the Pentagon -- scene of one of the strikes.

In New York, relatives of those killed when hijacked airliners slammed into the World Trade Center gathered at Ground Zero to remember the dead.

Former New York mayor Rudolph Giuliani wiped away a tear as the ceremony unfolded, starting with a moment of silence at 8:46 am (1246 GMT), when the first plane smashed into the Twin Towers.

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