NEW YORK • The United States yesterday commemorated the 15th anniversary of the Sept 11, 2001 attacks with a moment of silence observed in sombre remembrance at Ground Zero in New York, where nearly 3,000 people were killed.

The Al-Qaeda attacks killed 2,753 people in New York, 184 at the Pentagon in Washington, DC and 40 on Flight 93 - which had been heading towards the US capital when the passengers and crew staged a rebellion and the hijackers crashed the plane into a field in Pennsylvania.

US President Barack Obama, who marked the anniversary with a moment of silence in the Oval Office, was due to address a remembrance service at the Pentagon.

The first of what will be six moments of silence was observed at 8.46am (8.46pm Singapore time) - the time when the first hijacked passenger jet hit the North Tower of the World Trade Center.

Held at the Sept 11 memorial, the service also paused to mark the moment when the second plane hit the South Tower.

Other moments of silence were due to be observed at the time when each tower fell, as well as the attack on the Pentagon and Flight 93.

In New York, police and relatives of those killed in the World Trade Center began the annual reading of the names of the victims at Ground Zero, now the site of the National 9/11 Memorial and Museum.

"September 11, 2001 touched every single NYer but the terrorists did not prevail because, 15 years later, we are strong, and we are unified," New York Mayor Bill de Blasio wrote on Twitter.

As evening fell across the city yesterday, spotlights were to project two giant beams of light into the night sky to represent the fallen twin towers, fading away at dawn.

Presidential candidates Hillary Clinton and Donald Trump temporarily paused their bitter election campaign to attend the service with police and relatives of the victims at the Sept 11 memorial.

"We'll never forget the horror of Sept 11, 2001," Mrs Clinton said in a brief statement. "Let's honour the lives and tremendous spirit of the victims and responders."

Mr Trump said in a statement that it was a day of sadness and remembrance, but also of resolve.

AGENCE FRANCE-PRESSE, REUTERS