U.S. President Joe Biden attended a ceremony at the Pentagon on Sunday to mark the 21st anniversary of the 9/11 attacks.

On September 11, 2001, hijackers took control of four commercial airliners that they then crashed into New York's World Trade Center, the Pentagon and a Pennsylvania field, killing nearly 3,000 people.

"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.

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

The president 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. 

Jill Biden, the first lady, attended a ceremony in Pennsylvania on Sunday morning, while Vice President Kamala Harris attended one in New York City.

(With input from Reuters)