Biden recalls American unity, vows vigilance on 9/11 anniversary

US President Joe Biden has invoked the memory of America's united response to the September 11, 2001 attacks and vowed to "never give up" in the face of terrorist threats in a solemn commemoration at the Pentagon.

"I hope we'll remember that in the midst of these dark days, we dug deep. We cared for each other. And we came together," Biden said on Sunday on the 21st anniversary of the attacks, as rain fell on troops standing behind him.

Nearly 3,000 people died in the attacks, when al Qaeda hijackers flew planes into the New York's World Trade Centre towers and into the Pentagon in Arlington, Virginia, while a fourth plane crashed in Pennsylvania.

Passengers on United Airlines Flight 93 overcame the hijackers and the plane crashed in a field, preventing another target from being hit.

The 21st anniversary of the attacks comes a year after Biden ended the US-led invasion in Afghanistan, launched two decades ago to root out the Al Qaeda group that carried out the 9/11 attacks.

Biden vowed that the fight against terrorism would continue. "We will not rest. We'll never forget. We'll never give up," he said.

Last month, Biden authorised a drone strike in Kabul that killed al Qaeda leader Ayman al Zawahiri, an Egyptian surgeon who had a $25 million bounty on his head and helped to coordinate the September 11 attacks.

READ MORE: US marks 21st anniversary of 9/11 terror attacks

Biden stresses commitment

Biden and others argued that the threat from terrorism has spread throughout the world over the past 21 years, and that there are better ways to combat it than open-ended military deployments and war.

"Our commitment to prevent another attack on the United States is without end," Biden assured.

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 Defence 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.

The first lady, Jill Biden, attended a ceremony in Pennsylvania on Sunday morning, while Vice President Kamala Harris and second gentleman Doug Emhoff attended one in New York City.

Families of victims of the 9/11 attacks have waited for years to see several of the people accused of planning and assisting the hijackers brought to trial and convicted, including self-professed mastermind Khalid Sheikh Mohammed and four others imprisoned at the US military base in Guantanamo Bay, Cuba.

On Sunday, Biden told reporters that "yes, there is a plan for that" — to hold the accused plotters accountable — but declined to comment further.

READ MORE: FBI unveils first document linked to 9/11 attacks investigation