Along with First Lady Jill Biden, Biden saluted as a bugler played Taps, the US military funeral tune, at the sprawling five-sided building that is the seat of US military power.

Vice President Kamala Harris and her husband Doug Emhoff were also with the president as he laid a wreath at the Pentagon 9/11 memorial.

One of the planes hijacked by al-Qaeda members on September 11, 2001 slammed into one side of the Pentagon, killing 125 people.

Earlier Saturday, Biden attended memorial events in New York, where planes crashed into and eventually brought down the Twin Towers, and in Shanksville, Pennsylvania, where a commandeered jetliner crashed into a field after passengers overpowered the hijackers as the plane flew toward Washington.