Joe Biden commemorates 911 at three US sites

They then flew to Shanksville, Pennsylvania, and were scheduled to return to the Washington DC area and visit the Pentagon.

At the World Trade Center site, the New York Police Department pipes and drums band played Hard Times Come Again No More, a US folk song from the 1850s.

Bruce Springsteen, playing an acoustic guitar, sang I'll See You in My Dreams.

The Bidens, with former presidents Bill Clinton and Barack Obama and former first ladies Hillary Clinton and Michelle Obama, shared a moment of silence with the crowd at 8.46am to mark the time that the first plane hit.

Nearly 3000 people died in the attacks in New York, at the Pentagon and in Pennsylvania, where passengers on United Flight 93 overcame the hijackers and the plane crashed in a field, preventing another target from being hit.

In New York City, on a clear, beautiful day similar to the weather 20 years ago, relatives read a list of the people who died at the towers.

U.S. President Joe Biden attends the National 9/11 Memorial at the Pentagon in Washington. Source: AAP

Biden, head bowed, did not make remarks.

Rudy Giuliani, the mayor of New York at the time of the attacks, attended the ceremony.

In Shanksville, the Bidens participated in a wreath-laying ceremony at the Flight 93 National Memorial where names of the deceased are etched on a marble wall.

Vice President Kamala Harris said the passengers and crew members who died in Shanksville focused on common humanity during a time of terror.

"It is my hope and prayer that we continue to honour their courage, their conviction, with our own; that we honour their unity by strengthening our common bonds, by strengthening our global partnerships."

Later in the day Biden was 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 20 years ago to root out the al-Qaeda planners of the 9/11 attacks.

The events that led to 9/11

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

Biden was not scheduled to deliver remarks at any of the sites.

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 in late January but the country remains deeply divided politically.

US presidents often travel to one of the three sites on the 9/11 anniversary but it is unusual to go to all three 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 spokeswoman Jen Psaki said on Friday.

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.

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