US President Donald Trump observed a moment of silence Monday at a White House ceremony marking the 16th anniversary of the September 11, 2001 attacks that killed nearly 3,000 people.

The observance, along with another at Ground Zero in New York, was held at 8:46 am (1246 GMT), the moment the World Trade Center in Manhattan was struck by the first of two hijacked airliners.

In all, four planes were hijacked by Al-Qaeda militants who used them to topple the trade center's twin towers and hit the Pentagon. The fourth plane crashed in a field in Pennsylvania.

The attacks, the deadliest ever on US soil, killed 2,997 people, and plunged the United States into a rolling chain of wars against Islamist militants.