"I authorized a precision strike that would remove him from the battlefield, once and for all," CNN quoted Biden as saying.

Zawahiri, Biden said, "was deeply involved in the planning of 9/11, one of the most responsible for the attacks that murdered 2,977 people on American soil. For decades, he was the mastermind of attacks against Americans."

"Now, justice has been delivered and this terrorist leader is no more. People around the world no longer need to fear the vicious and determined killer," he continued. "The United States continues to demonstrate our resolve and our capacity to defend the American people against those who seek to do us harm. We make it clear again tonight, that no matter how long it takes, no matter where you hide, if you are a threat to our people, the United States will find you and take you out," he added.

Zawahiri, who just turned 71 years old, had remained a visible international symbol of the group, 11 years after the US killed Osama bin Laden. At one point, he acted as bin Laden's personal physician.

Zawahiri was sheltering in downtown Kabul to reunite with his family, Biden said, and was killed in what a senior administration official described as "a precise tailored airstrike" using two Hellfire missiles.

The drone strike was conducted at 9:48 p.m. ET on Saturday was authorized by Biden following weeks of meetings with his Cabinet and key advisers, the official said on Monday, adding that no American personnel were on the ground in Kabul at the time of the strike.