In a somber ceremony at the Pentagon, Vice President Joe Biden recalled America's united reaction to the September 11, 2001, terrorist attacks and vowed to "never give up" in the face of terrorist threats.

Biden remarked this on the 21st anniversary of the assaults, as rain fell on troops standing behind him.

Al Qaeda hijackers drove planes into the World Trade Center towers in New York and the Pentagon in Arlington, Virginia, while a fourth plane crashed in Pennsylvania, killing over 3,000 people.

United Airlines Flight 93 passengers overpowered the hijackers and caused the plane to crash in a field, so preventing another target from being struck.

The 21st anniversary of the attacks occurs one year after Vice President Biden halted the US-led operation in Afghanistan, which was undertaken two decades ago to root out the Al Qaeda cell responsible for 9/11.

Biden committed to continuing the fight against terrorism. He stated, "We will not rest. We'll never forget. We'll never give up,"

Al Qaeda commander Ayman al Zawahiri, an Egyptian surgeon with a $25 million bounty on his head who helped orchestrate the September 11 attacks, was killed by a drone hit in Kabul last month.

Biden stresses commitment

Biden and others claimed that the threat of terrorism has extended globally over the past 21 years and that there are better ways to tackle it than open-ended military deployments and conflict.

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

During the yearly moment of remembrance held in New York City, the Pentagon, and Somerset County, Pennsylvania, the president was joined by family members of the slain, first responders who were present at the Pentagon on the day of the assault, and the Defence Department leadership.

Biden stated, "We owe you an incredible, incredible debt."

On Sunday morning, the first lady, Jill Biden, attended a ceremony in Pennsylvania, while the vice president and second gentleman attended a ceremony in New York City.

Families of 9/11 victims have waited years to see a number of those accused of organizing and assisting the hijackers brought to trial and convicted, including self-proclaimed mastermind Khalid Sheikh Mohammed and four others held at the US military camp in Guantanamo Bay, Cuba.

Biden told reporters on Sunday that there is a strategy to bring the suspected conspirators accountable, but he declined to go further.