US President Barack Obama will hold a memorial service at the Pentagon Saturday to mark the ninth anniversary of the worst terrorist attack ever on the US, the White House said Thursday.

US Vice President Joe Biden will attend a ceremony at the former site of the World Trade Centre, where the twin towers collapsed after being struck by hijacked commercial airliners Sep 11, 2001.

Moments later another hijacked airliner slammed into the Pentagon and a fourth crashed in a Pennsylvania field after the passengers tried to retake control of the plane from the terrorists.

The attacks killed 2,973 people plus the 19 hijackers.