The United States on Sunday observed the 21st anniversary of the 9/11 attacks, with President Joe Biden recalling Americans’ sacrifice and with New Yorkers honouring the nearly 3,000 people killed when hijacked planes destroyed the Twin Towers in their city.

The attacks killed nearly 3,000 people and spurred a so-called US “war on terror”. More than a million people are estimated to have died following the US military intervention in several Muslim countries as part of “the war on terror”.

Relatives of victims, police officers, firefighters and city leaders gathered at the National September 11 Memorial and Museum in Lower Manhattan, where the names of those who died were read aloud in an hours-long ritual that has occurred annually since the deadliest single attack on US soil.

They rang bells and held moments of silence six times, including at 8:46am and 9:03am (12:46 and 13:03 GMT), the precise minutes the passenger jets struck the World Trade Center’s North and South Towers.

Biden commemorated the anniversary at the Pentagon, where al-Qaeda hijackers crashed a plane into the massive building that serves as US Department of Defense headquarters.

In a steady rain, the president approached a wreath of flowers and placed his hand over his heart.

“I know for all those of you who lost someone, 21 years is both a lifetime and no time at all,” Biden said in a sombre speech outside the Pentagon.

“The American story itself changed that day,” he said. “What we cannot change, never will, is the character of this nation.”

“The character of sacrifice and love, of generosity and grace, of strength and resilience,” he said.

Biden also recalled how in the hours after the attacks, Queen Elizabeth II – who died Thursday at age 96 – sent a touching message to the American people.

“She pointedly reminded us: ‘Grief is the price we pay for love,'” Biden said.

Al-Qaeda hijacked a total of four planes. The third hit the Pentagon and the fourth, Flight 93, crashed in a field near Shanksville, Pennsylvania after passengers launched a revolt against the hijackers onboard.