WASHINGTON, Aug 1 (Reuters) - The United States killed al Qaeda leader Ayman al-Zawahiri in a strike in Afghanistan over the weekend, President Joe Biden said on Monday, the biggest blow to the militant group since its founder Osama bin Laden was killed in 2011.

Zawahiri, an Egyptian surgeon who had a $25 million bounty on his head, helped coordinate the Sept. 11, 2001, attacks that killed nearly 3,000 people.

U.S. officials, speaking on the condition of anonymity, said Zawahiri died after a U.S. drone strike in the Afghan capital Kabul at 6:18 a.m. (0148 GMT) on Sunday.

"Now justice has been delivered, and this terrorist leader is no more," Biden, who is recovering from COVID-19, said in remarks from the White House. "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."

U.S. intelligence determined with "high confidence" through multiple intelligence streams that the man killed was Zawahiri, a senior administration official told reporters. He was killed on the balcony of a "safe house" in Kabul that he shared with other members of his family. No other casualties occurred.

Biden said Zawahiri had been the mastermind behind or played a key role in attacks on the USS Cole and U.S. embassies in Kenya and Tanzania.

"Zawahiri continued to pose an active threat to U.S. persons, interests and national security," the official said on a conference call. "His death deals a significant blow to al Qaeda and will degrade the group's ability to operate."

There were rumors of Zawahiri's death several times in recent years, and he was long reported to have been in poor health.

His death raises questions about whether Zawahiri received sanctuary from the Taliban following their takeover of Kabul in August 2021. The official said senior Taliban officials were aware of his presence in the city.

The drone attack is the first known U.S. strike inside Afghanistan since U.S. troops and diplomats left the country in August 2021. The move may bolster the credibility of Washington's assurances that the United States can still address threats from Afghanistan without a military presence in the country.

In a statement, Taliban spokesman Zabihullah Mujahid confirmed that a strike took place and strongly condemned it, calling it a violation of "international principles."

Zawahiri succeeded bin Laden as al Qaeda leader after years as its main organizer and strategist, but his lack of charisma and competition from rival militants Islamic State hobbled his ability to inspire spectacular attacks on the West.