The US has killed al-Qaeda leader Ayman al-Zawahiri in a strike in Afghanistan, President Joe Biden said, the biggest blow to the militant group since its founder Osama bin Laden was killed in 2011.

Zawahiri, an Egyptian surgeon who had a $25m (€24m) bounty on his head, helped co-ordinate the 11 September 2001 attacks that killed nearly 3,000 people.

US officials, speaking on the condition of anonymity, said Zawahiri died after a US drone strike in the Afghan capital Kabul at 6.18 am local time yesterday.

"Now justice has been delivered, and this terrorist leader is no more," Mr 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."

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US intelligence determined with "high confidence" through multiple intelligence streams that the man killed was Zawahiri, one 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. There were no other casualties.

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

"Zawahiri continued to pose an active threat to US 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 rumours of Zawahiri's death several times in recent years, and he was long reported to have been in poor health.

The drone attack is the first known US strike inside Afghanistan since US 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.

His death also raises questions about whether Zawahiri received sanctuary from the Taliban following its takeover of Kabul in August 2021.

The official said senior Taliban officials were aware of his presence in the city and said the United States expected the Taliban to abide by an agreement not to allow al-Qaeda fighters to re-establish themselves in the country.

"The Taliban will have to answer for al-Zawahiri's presence in Kabul, after assuring the world they would not give safe haven to al-Qaeda terrorists," Adam Schiff, chairman of the House Permanent Select Committee on Intelligence, said in a statement.

Joe Biden confirmed the reports in remarks from the White House

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 organiser and strategist, but his lack of charisma and competition from rival militants Islamic State hobbled his ability to inspire devastating attacks on the West.

'Support of the Taliban is really quite worrisome'

Former director of the CIA John Brennan has said that while Ayman al-Zawahiri had been in hiding in northern Pakistan for many years, he was "still at the helm" of the al-Qaeda organisation.

Speaking to RTÉ's Drivetime, Mr Brennan said: "He wasn't very much engaged in operational activities over the past decade because of the lack of the opportunity to communicate with al-Qaeda operatives that still exist in many parts of the world, including in Afghanistan, Pakistan, Yemen, and other areas.

"The fact that he was able to travel to Afghanistan and to Kabul, under the guidance and safe haven and support of the Taliban is really quite worrisome, because it would then allow al-Zawahiri to have the types of communication with al-Qaeda members that he was not able to have from northern Pakistan, so I think there are a lot of questions about what he was doing in Kabul and whether or not he was going to try to reconstitute al-Qaeda's worldwide terrorist networks."

He said that this raises questions about the Taliban and its pledge that it would not allow Afghanistan to be used by terrorist organisations to carry out terrorist attacks abroad.

'The world is safer without him in it'

Republican and Democratic lawmakers have lauded the operation.

"The world is safer without him in it and this strike demonstrates our ongoing commitment to hunt down all terrorists responsible for 9/11 and those who continue to pose a threat to US interests," said Republican US Senator Marco Rubio.

Until the US announcement, Zawahiri had been rumoured to be in Pakistan's tribal area or inside Afghanistan.

A video released in April in which he praised an Indian Muslim woman for defying a ban on wearing an Islamic head scarf dispelled rumours that he had died.

The senior US official said finding Zawahiri was the result of persistent counterterrorism work.

The United States identified this year that Zawahiri's wife, daughter and her children had relocated to a safe house in Kabul, then identified that Zawahiri was there as well, the official said.

"Once Zawahiri arrived at the location, we are not aware of him ever leaving the safe house," the official said.

He was identified multiple times on the balcony, where he was ultimately struck. He continued to produce videos from the house and some may be released after his death, the official said.

In the last few weeks, Mr Biden convened officials to scrutinise the intelligence. He was updated throughout May and June and was briefed on 1 July on a proposed operation by intelligence leaders.

On 25 July he received an updated report and authorised the strike once an opportunity was available, the administration official said.

A loud explosion echoed through Kabul early yesterday morning.

"A house was hit by a rocket in Sherpoor. There were no casualties as the house was empty," Abdul Nafi Takor, spokesman of the interior ministry, said earlier.

One Taliban source, requesting anonymity, said there had been reports of at least one drone flying over Kabul that morning.

With other senior al-Qaeda members, Zawahiri is believed to have plotted the 12 October 2000 attack on the USS Cole naval vessel in Yemen which killed 17 US sailors and injured more than 30 others, the Rewards for Justice website said.

He was indicted in the United States for his role in the 7 August 1998, bombings of the US embassies in Kenya and Tanzania that killed 224 people and wounded more than 5,000 others.

Both bin Laden and Zawahiri eluded capture when US-led forces toppled Afghanistan's Taliban government in late 2001 following the 11 September attacks on the United States.