In a pre-recorded message to mark the 20th anniversary of 11 September terror attacks, US President Joe Biden remembered the events as a moment defined not only by heroism but "unity and resilience." "We learned that unity is the one thing that must never break," Biden said in a six-minute video filmed at the White House, news wires reported. "At our most vulnerable, in the push and pull of all that makes us human, in the battle for the soul America, unity is our greatest strength," he stressed.

Biden is set to visit New York City, Shanksville, PA., and the Pentagon - all three sites where planes crashed - on Saturday to commemorate the 20 years since the attack. The president, who will lay wreaths and pay tribute to those who lost their lives, is not due to give remarks at any of the three events.

White House press secretary Jen Psaki said none of the events include a "lengthy speaking program" which is why the president is delivering a message ahead of the anniversary.

"Unity doesn't mean we have to believe in the same thing," Biden said in an apparent reference to the country's political divisions. "But we must have a fundamental respected faith in each other and in this nation."

Biden will join former President George W. Bush, who was commander in chief during the 2001 terrorist attacks, in Shanksville, while former President Barack Obama will attend the remembrance ceremony at Ground Zero in New York.

Nearly 3,000 people were killed in the terrorist attacks in New York, Washington, D.C., and Pennsylvania.

The anniversary comes more than a week after the US completed its withdrawal from Afghanistan, ending a 20-year occupation that began after the 9/11 terrorist attacks.

Earlier this month Biden signed an executive order directing the Department of Justice and other federal agencies to declassify material related to the 9/11 attacks for possible release of sensitive information – meeting a demand of victims' families before the 20-year anniversary.

Relatives of Americans who were killed on 9/11 have spent years pushing successive administrations to release classified information that they said would reveal Saudi Arabia’s complicity in the attacks. Some had even said they did not want Biden to attend any memorial events unless he moved to release the long-sought documents.