US President Joe Biden participates in a wreath laying ceremony to pay his respects to the victims of the 9/11 attacks, at the Pentagon in Washington, DC, on Sunday. AFP

Sunday's ceremony occurred a little more than a year after Biden ended the long and costly war in Afghanistan that the U.S. and allies launched in response to the terror attacks.

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US marks 21st anniversary of 9/11 terror attacks

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Biden noted that even after the United States left Afghanistan that his administration continues to pursue those responsible for the 9/11 attacks. Last month, Biden announced the US had killed Ayman Al Zawahri, the Al Qaida leader who helped plot the Sept. 11 attacks, in a clandestine operation.

US President Joe Biden delivers remarks at the National 9/11 Pentagon Memorial in Arlington, Virginia. AFP

"We will never forget, we will never give up,” Biden said. "Our commitment to preventing another attack on the United States is without end.”

The president 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 Defense Department leadership for the annual moment of tribute carried out in New York City, the Pentagon and Somerset County, Pennsylvania.

"We owe you an incredible, incredible debt,” Biden said.

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.

Biden marked the one-year anniversary of the US withdrawal from Afghanistan late last month in low-key fashion. He issued a statement in honor 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.

Flowers are visible on one of the benches at the National 9/11 Pentagon Memorial in Washington. AP

Senate Minority Leader Mitch McConnell on Thursday criticised Biden's handling of the end of the war and noted that the country has spiraled 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," 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."

The president also remembered the words of comfort Queen Elizabeth II, who died last week, sent to the American people soon after the 2001 attacks: "Grief is the price we pay for love." Biden said those words remain as poignant as they did 21 years ago but the weight of loss also remains heavy.

"On this day, the price feels so great,” Biden said.