WASHINGTON: 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, reports AFP.

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. The president, and his wife, Melania, placed their hands on their hearts and bowed their heads as a bell tolled and a Marine played the mournful Taps on a trumpet.

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 chain of rolling wars against Islamic militants.

Memorial ceremonies were also being held in New York and at the Pentagon and Vice President Mike Pence was to deliver remarks in Shanksville, Pennsylvania, where the fourth plane crashed. Pence was also scheduled to tour the Flight 93 National Memorial Visitors Center and take part in a wreath-laying ceremony. Yet last year's 15th-anniversary ceremony became entangled in the narrative of a fractious presidential campaign when Democratic nominee Hillary Clinton left abruptly, stumbled into a van and ultimately revealed she'd been diagnosed days earlier with pneumonia.

The episode fed into questions that then-Republican-nominee Trump had repeatedly raised about Clinton's stamina and transparency. She took three days off to recover, and Trump used footage of her stagger in a campaign ad.

Trump has often invoked his memories of 9/11 to highlight his hometown's resilience and responders' bravery. Some of his recollections have raised eyebrows, particularly remarks while talking about Muslims that "thousands of people were cheering" in Jersey City, New Jersey, as the towers fell.