President Joe Biden called Monday for Americans to unite despite bitter political differences as the United States marked the 22nd anniversary of Al-Qaeda's 9/11 attacks.

Bells were rung and the names of nearly 3,000 people were read out in somber ceremonies in New York, Washington, and Pennsylvania where the hijacked planes struck.

"Let's honor September 11 by renewing our faith in one another," said Biden, speaking at a US military base in Anchorage, Alaska as he traveled back from a trip to India and Vietnam.

"We must never lose our sense of national unity, so let that be the common cause of our time."

Speaking in front of a huge flag, Biden added that "terrorism, including political and ideological violence, is the opposite of all we stand for as a nation."

His speech comes as the United States is increasingly polarized, with tensions likely to increase as Biden, a Democrat, heads into a likely election rematch next year with Republican former president Donald Trump.

Trump has been indicted four times since April, including for efforts to overturn the 2020 election results, with the 6 January 2021 Capitol attack by his supporters still fresh in the public's memory.

'Never forget'

In New York, Vice President Kamala Harris and current and former mayors joined victims' families at the 9/11 memorial on the site of the World Trade Center twin towers brought down by two aircraft flown by hijackers.

The names of the more than 2,600 who died in New York were read out by family members and young relatives not alive at the time of the attack.

"I wish I had a chance to really know you. Everyone in the family misses you. We will never forget," said the grandson of firefighter Allan Tarasiewicz, who was killed at age 45 during rescue operations at the World Trade Center.

At the Pentagon in Washington, where the attackers plunged a third aircraft into the headquarters of the US military, a sailor rang a ship's bell for each of the 184 killed there.