Moment of silence for 9/11 attack victims

The commemoration of the 18th anniversary of the September 11 terror attacks has begun at ground zero in New York with a moment of silence and tolling bells.

Victims' relatives and dignitaries were gathered on the memorial plaza at the World Trade Centre as the ceremony started at 8.46am on Wednesday.

That is the time when a hijacked plane slammed into the World Trade Centre's north tower on September 11, 2001.

Victims' loved ones began reading the names of the nearly 3000 people killed when a total of four hijacked planes crashed into the trade centre, the Pentagon and a Pennsylvania field.

President Donald Trump is attending an observance at the Pentagon. Vice President Mike Pence is speaking at the 9/11 memorial near Shanksville, Pennsylvania.