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.

And in western Pennsylvania, where a fourth hijacked plane apparently heading toward Washington was forced to crash, bells were rung for each of the 40 passengers and crew who died.

"September 11 made America a nation at war, and hundreds of thousands stepped up to serve our country in uniform," Defense Secretary Lloyd Austin said at the Pentagon ceremony.

"I know that it aches to remember this milestone year after year… The men and women of the Department of Defense will always remember."

Across New York City, in Congress and elsewhere, a moment of silence was held to mark the attack, plotted by Al-Qaeda chief Osama bin Laden, who was found and killed nearly a decade later by US Navy Seals in a raid on his hideout in Pakistan.

Biden noted in his speech that he himself had given the order for bin Laden's successor Ayman al-Zawahiri to be sent to the "gates of hell" last year in an airstrike in Afghanistan.

"The soul of America is the fortitude we found in the fear of that terrible September day," he added.

"The terrorists believed they could bring us to our knees, bend our will, break our resolve. But they were wrong, they were dead wrong."