President Joe Biden will commemorate the 20th anniversary of the Sept 11 attacks on the United States on Saturday by visiting each of the sites where hijacked planes crashed in 2001, seeking to honor the victims of the devastating assault.

Biden will begin the day in New York, where he will attend a ceremony at 8:30 EDT at the site where the World Trade Center's twin towers once stood before planes struck the two buildings and caused them to collapse.

Then he will travel to Shanksville, Pennsylvania, where United Flight 93 crashed into a field after passengers overtook the hijackers and prevented another target from being hit.

Finally Biden will return to the Washington area to visit the Pentagon, the symbol of US military might that was pierced by another of the planes that were used as missiles that day.

The anniversary comes shortly after the end of the US-led war in Afghanistan, launched some 20 years ago to root out al Qaeda, which carried out the 9/11 attacks.

Biden's withdrawal of US troops in August, months after a deadline set by his Republican predecessor, Donald Trump, and the resulting rapid fall of the country to the Taliban has drawn criticism from members of both political parties.

Biden is not scheduled to deliver remarks at any of the sites, but he released a video on Friday to express his condolences to the loved ones of the victims and highlight the national unity that resulted, at least initially, after 9/11.

"It's so hard. Whether it's the first year or the 20th, children have grown up without parents and parents have suffered without children," Biden said.

The president noted the heroism that was seen in the days following the attacks.

"We also saw something all too rare: a true sense of national unity," Biden said.

Biden, a Democrat, pledged to build up such unity after he came into office earlier this year, but the country remains deeply divided politically.

US presidents often travel to one of the three attack sites on the 9/11 anniversary but it is unusual to go to all three of them on the same day.

"The president felt it was important to visit each of these three sites to commemorate the lives lost, the sacrifices made on a day that has impacted millions of people across the country but certainly many people in those communities," White House spokesperson Jen Psaki said on Friday.

Biden is comfortable in the role of consoler-in-chief after suffering tragic family losses himself.

"No matter how much time has passed, and these commemorations bring everything painfully back, as if you just got the news a few seconds ago. And so on this day, Jill and I hold you close in our hearts and send you our love," he said in the video remarks, referencing his wife, Jill Biden.

Last month, many families of 9/11 victims asked Biden to skip 20-year memorial events unless he declassified documents they contend will show Saudi Arabian leaders supported the attacks. Last week the president ordered the Department of Justice to review documents from the FBI probe into the attacks for declassification and release.