In a video released on the eve of the 20th anniversary, he paid tribute to the 2,977 people who lost their lives.

"We honour all those who risked and gave their lives in the minutes, hours, months and years afterwards," Mr Biden added, speaking of the emergency workers who responded to the attacks.

Commemoration events are due to take place on Saturday.

He acknowledged the "darker forces of human nature - fear and anger, resentment and violence against Muslim Americans" which followed the attacks, but said that unity had remained the US' "greatest strength".

"We learned that unity is the one thing that must never break," he added.

This is the first major anniversary of the attacks where a generation of people has grown up with no recollection of the events that day.

Four planes flying over the eastern US were seized simultaneously by small teams of suicide attackers. Thousands were murdered in a matter of minutes.

The planes were used as guided missiles to crash into landmark buildings in New York and Washington.

Two planes struck the Twin Towers of the World Trade Center in Manhattan and a third plane hit the Pentagon - the US military headquarters just outside Washington DC.

The fourth plane crashed in a field in Pennsylvania after passengers fought back against their hijackers.

In all, 2,977 people (not counting the 19 hijackers) lost their lives, most of them in New York.