United States marks 15th anniversary of 9/11

Nearly 3,000 people were killed on September 11, 2001 when 19 Al-Qaeda suicide bombers hijacked four passenger jets, crashing them into the Twin Towers in New York, the Pentagon in Washington and a field in Shanksville, Pennsylvania.

It was the first foreign attack on the US mainland in almost two centuries and sparked US-led invasions of Afghanistan (2001) and Iraq (2003, where war rages on more than a decade later.

Countries in the Middle East from Libya to Syria are engulfed in war, providing a fertile ground for Al-Qaeda affiliates to breed, and Europe has been tormented by attacks inspired by the Islamic State group.

The names of the dead will be read out in a remembrance service at Ground Zero in New York on the site of the rebuilt World Trade Center, and President Barack Obama will address a ceremony at the Pentagon.

The 15th anniversary comes with the United States in the midst of a divisive presidential campaign between Hillary Clinton, who was New York senator at the time of the attacks, and Manhattan tycoon Donald Trump. 

Held at the September 11 memorial, the service will pause six times -- to mark the moments when each of the two planes hit, when each tower fell, as well as the attack on the Pentagon and Flight 93 that crashed in Pennsylvania.

During the first moment of silence at 8:46 am, the time when American Airlines Flight 11 hit the north tower, houses of worship in New York have been asked to toll their bells.

President Barack Obama will observe a moment of silence at the White House before addressing a memorial ceremony at the Pentagon.

George W. Bush, who was president at the time of the attacks, is not expected to attend either ceremony in New York or Washington.

The attacks killed 2,753 people in New York, 184 at the Pentagon and 40 on Flight 93 -- which was headed toward the US capital until passengers and crew staged a rebellion and the hijackers crashed it.