WATCH: Biden Marks 20th Anniv. of 9/11 Attacks with Heartfelt Message

U.S. President Joe Biden marked the 20th anniversary of the Sept. 11 terror attacks through a heartfelt message in a pre-recorded video.

"We learned that unity is the one thing that must never break," Biden said in a six-minute video filmed at the White House. "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."

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

The president 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. He will lay wreaths and pay tribute to those who lost their lives.

Moreover, he is going to 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.

On her part, 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.

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 U.S. completed its withdrawal from Afghanistan, ending a 20-year occupation that began after the 9/11 terrorist attacks.