President Joe Biden announced Monday that the United States had killed Al-Qaeda chief Ayman al-Zawahiri, one of the world's most wanted terrorists and a mastermind of the September 11, 2001 attacks, in a drone strike in Kabul.

In a somber televised address, Biden said he gave the final go-ahead for the high-precision strike that successfully targeted Zawahiri in the Afghan capital over the weekend.

"Justice has been delivered and this terrorist leader is no more," Biden said, adding that he hoped Zawahiri's death would bring "closure" to families of the 3,000 people killed in the United States on 9/11.

A senior administration official said Zawahiri was on the balcony of a house in Kabul when he was targeted with two Hellfire missiles, an hour after sunrise on July 31, and that there had been no US boots on the ground in Afghanistan.

"We are not aware of him ever leaving the safe house. We identified Zawahiri on multiple occasions for sustained periods of time on the balcony of where he was ultimately struck," the official said.

According to the official's account, the president gave his green light for the strike on July 25 -- as he was recovering in isolation from Covid-19. Biden said there were no civilian casualties in the operation.

The US official called Zawahiri's presence in the Afghan capital Kabul a "clear violation" of a deal the Taliban had signed with the US in Doha in 2020 that paved the way for the US withdrawal from Afghanistan.

It was the first known over-the-horizon strike by the United States on an Al-Qaeda target in Afghanistan since American forces withdrew from the country on August 31, 2021.

Zawahiri, an Egyptian surgeon who grew up in a comfortable Cairo household before turning to violent radicalism, had been on the run for 20 years since the 9/11 attacks.

He took over Al-Qaeda after Osama bin Laden was killed by US special forces in Pakistan in 2011, and had a $25 million US bounty on his head.

Over the weekend the Afghan interior ministry denied reports circulating on social media of a drone strike in Kabul, telling AFP a rocket struck "an empty house" in the capital, causing no casualties.

Early Tuesday in Kabul, however, Taliban spokesman Zabihullah Mujahid tweeted that an "aerial attack" was carried out on a residence in the Sherpur area of the city.

"The nature of the incident was not revealed at first. The security and intelligence agencies of the Islamic Emirate investigated the incident and found in their preliminary investigations that the attack was carried out by American drones," his tweet said.

The news comes a month before the first anniversary of the final withdrawal of US troops from Afghanistan, leaving the country in the control of the Taliban insurgency that fought Western forces over the preceding two decades.

Under the 2020 Doha deal, the Taliban promised not to allow Afghanistan to be used again as a launchpad for international jihadism, but experts believe the group never broke their ties with Al-Qaeda.

"What we know is that the senior Haqqani Taliban were aware of his presence in Kabul," the Biden official said.

Taliban interior minister Sirajuddin Haqqani also heads the feared Haqqani Network, a brutal subset of the Taliban blamed for some of the worst violence of the past 20 years and which has been described by US officials as a "veritable arm" of Pakistani intelligence.

Doctor turned jihadist

Zawahiri, 71, lacked the potent charisma that helped bin Laden rally jihadists around the world, but willingly channelled his analytical skills into the Al-Qaeda cause.

He was believed to be the main strategist -- the real mastermind who steered operations, including the September 11 attacks, as well as bin Laden's personal doctor.

Al-Qaeda is believed to have been degraded in the years since the US invasion of Afghanistan, and the White House official said Zawahiri was "one of the last remaining figures who carried this kind of significance."

The organization, agreed Soufan Center researcher Colin Clarke, is "at a crossroads."

"Despite Zawahiri's leadership, which minimized AQ's losses while rebuilding, the group still faces serious challenges going forward. For one, there's the question of who will lead al Qaeda after Zawahiri's gone," he said.

Zawahiri's father was a renowned physician and his grandfather a prayer leader at Cairo's Al-Azhar institute, the highest authority for Sunni Muslims.

He became involved with Egypt's radical Muslim community at a young age and published several books which came for many to symbolize the radical Islamist movement.

He left Egypt in the mid-1980s, heading for Pakistan's northwestern city of Peshawar where the resistance to the Soviet occupation of Afghanistan was based.

It was at that time, when thousands of Islamist fighters flooded into Afghanistan during the 1980s, that Zawahiri and bin Laden met, and in 1998 he became one of five signatories to bin Laden's "fatwa" calling for attacks against Americans.

Jihadist monitor SITE said some militants were questioning the veracity of the report he had been killed, while others believed Zawahiri had achieved his desire of "martyrdom."

As for Al-Qaeda's future without him, SITE said jihadists were bullish, with one writing: "If Sheikh Ayman al-Zawahiri is dead, there are a thousand Aymans."