US President Joe Biden has marked the 21st anniversary of the September 11 attacks, taking part in a sombre wreath-laying ceremony at the Pentagon held under a steady rain and paying tribute to “extraordinary Americans” who gave their lives on one of the country’s darkest days.

Sunday’s ceremony occurred a little more than a year after Mr Biden ended the long and costly war in Afghanistan that the US and allies launched in response to the attacks in New York, at the Pentagon and in Pennsylvania.

Mr Biden said that even after United States forces left Afghanistan his administration continues to pursue those responsible for the 9/11 attacks.

Last month, Mr Biden announced the US had killed Ayman al-Zawahri, the al-Qaeda leader thought to have helped plot the September 11 attacks.

“We will never forget, we will never give up,” Mr Biden said.

“Our commitment to preventing another attack on the United States is without end.”

Mr Biden was joined by family members of the fallen, first responders who had been at the Pentagon on the day of the attack, as well as Defence Department leadership for the annual moment of tribute carried out in New York City, the Pentagon and Somerset County, Pennsylvania.

In ending the Afghanistan war, the Democratic president followed through on a campaign pledge to bring home US troops from the country’s longest conflict.

But the war concluded chaotically in August 2021, when the US-backed Afghan government collapsed, a grisly bombing killed 170 Afghans and 13 US troops at Kabul’s airport, and thousands of desperate Afghans gathered in hopes of escape before the final US cargo planes departed over the Hindu Kush.

Mr Biden marked the one-year anniversary of the US withdrawal from Afghanistan late last month in low-key fashion.

He issued a statement in honour of the 13 US troops killed in the bombing at the Kabul airport and spoke by phone with US veterans assisting ongoing efforts to resettle in the United States Afghans who helped the war effort.

Mr Biden on Sunday said an “incredible debt” was owed to the US troops who served in Afghanistan as well as their families.

More than 2200 US service members were killed and more than 20,000 were wounded over the course of the nearly 20-year war, according to the Pentagon.

He also vowed that the US will “never fail to meet the sacred obligation to you to properly prepare and equip those that we send into harm’s way and care for those and their families when they come home – and to never, ever, ever forget”.

Senate Minority Leader Mitch McConnell on Thursday criticised Biden’s handling of the end of the war and noted that the country has spiralled downward under renewed Taliban rule since the US withdrawal.

“Now, one year on from last August’s disaster, the devastating scale of the fallout from President Biden’s decision has come into sharper focus,” Mr McConnell said.

“Afghanistan has become a global pariah. Its economy has shrunk by nearly a third. Half of its population is now suffering critical levels of food insecurity.”

First lady Jill Biden spoke to a crowd at the Flight 93 National Memorial Observance in Shanksville, Pennsylvania, where she recalled the concern she had about her sister Bonny Jacobs, a United Airlines flight attendant.

She said the attacks showed that “with courage and kindness we can be a light in that darkness”.