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(AFP) - A United States drone strike killed Al-Qaeda chief Ayman al-Zawahiri at a hideout in Kabul, President Joe Biden said Monday, declaring that "justice had been delivered" to the families of the 9/11 attacks.

Zawahiri's assassination is the biggest blow to Al-Qaeda since US special forces killed Osama bin Laden in 2011, and calls into question the Taliban's promise not to harbour militant groups.

It was the first known over-the-horizon strike by the US on a target in Afghanistan since Washington withdrew its forces from the country on August 31 last year, days after the Taliban swept back to power.

"Justice has been delivered and this terrorist leader is no more," Biden said in a sombre televised address, adding he hoped Zawahiri's death would bring "closure" to families of the 3,000 people killed in the US on September 11, 2001.

Zawahiri was believed to be the mastermind who steered Al-Qaeda's operations -- including the 9/11 attacks -- as well as bin Laden's personal doctor.

A senior administration official said the 71-year-old Egyptian was on the balcony of a three-storey house in the Afghan capital when targeted with two Hellfire missiles after dawn Sunday.

"We identified Zawahiri on multiple occasions for sustained periods of time on the balcony where he was ultimately struck," the official said.

The house is in Sherpur, one of Kabul's most affluent neighbourhoods, with several villas occupied by high-ranking Taliban officials and commanders.

The interior ministry previously denied reports of a drone strike circulating on social media, telling AFP a rocket struck "an empty house" in Kabul, causing no casualties.

Early Tuesday, however, Taliban spokesman Zabihullah Mujahid tweeted that an "aerial attack" was carried out.

"The nature of the incident was not revealed at first," he said.

"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."

- 'Grossly violated' -

Although Biden did not mention the Taliban in his televised address, Secretary of State Antony Blinken said "by hosting and sheltering" Zawahiri, the Islamist group had "grossly violated the Doha Agreement" which paved the way for America's withdrawal.

Zabihullah, in turn, accused Washington of breaking the 2020 deal.

"Such actions are a repetition of the failed experiences of the past 20 years and are against the interests of the United States of America, Afghanistan, and the region," he said.

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

He took over Al-Qaeda after bin Laden was killed, and had a $25 million US bounty on his head.

News of his death comes a month before the first anniversary of the final withdrawal of US troops from Afghanistan, leaving the country in the hands of the Taliban insurgency that fought Western forces for two decades.

Under the 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 ties with Al-Qaeda.

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

Afghan 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.

In Sherpur, locals told AFP they long thought the targeted house -- surrounded by high walls and barbed wire, with green tarpaulin covering the balcony where Zawahiri was believed to have been killed -- was empty.

"We have not seen anybody living there for almost a year," said an employee of a nearby office.

"It has always been in dark, with not a single bulb lit."

- Doctor turned jihadist -

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

Still, the group is believed to have been degraded 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 organisation is "at a crossroads", said Soufan Center researcher Colin Clarke.

"Despite Zawahiri's leadership, which minimised 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."

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 Islamist community at a young age and published several books which came for many to symbolise the 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.

Thousands of Islamist fighters were flooding into Afghanistan at the time, setting the stage for Zawahiri's first meeting with bin Laden.

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."

© Agence France-Presse