New York - The United States on Sunday commemorated the 15th anniversary of the 9/11 attacks with a moment of silence observed in sombre 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 a moment of silence in the Oval Office, will address a remembrance service at the Pentagon.
The first of what will be six moments of silence was observed at 08:46 (12:46 GMT) - 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.