NEW YORK (AFP) - The United States on Sunday (Sept 11) commemorated the 15th anniversary of the 9/11 attacks with a moment of silence observed in somber remembrance at Ground Zero in New York, where nearly 3,000 people were killed.

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 September 11 memorial.

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 also been headed toward the US capital until passengers and crew staged a rebellion and the hijackers crashed it into a field in Pennsylvania.

President Barack Obama, who marked the anniversary with his family in the residence of the White House, will address a remembrance service at the Pentagon.

"This weekend, we honour their memory once more. We stand with the survivors who still bear the scars of that day," Mr Obama said on Saturday in his weekly address. "In the face of terrorism, how we respond matters. We cannot give in to those who would divide us. We cannot react in ways that erode the fabric of our society."

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 September 11 memorial, the service also paused to mark the moment when the second plane hit the South Tower. Other moments of silence will take place 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.