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 sombre 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.

Here are five details on the security operation that culminated in the killing of Al-Qaida's al-Zawahiri:

1. The Hellfire R9X first appeared in 2017 when Al-Qaeda senior leader Abu al-Khayr al-Masri was killed, according to a report by news agency AP. What suggests the use of the missile is that pictures showed no signs of explosion , the report highlighted. Over time, the missile has earned names like “flying ginsu," after a famous 1980s television commercial for ostensibly Japanese kitchen knives, and “ninja bomb”.

2. In his address, Biden pointed out that there was no harm done to the family members of Zawahiri in the precision strike, and no civilians bore any injuries.

3. Senior Taliban officials were aware of Ayman al-Zawahiri's presence in Kabul, news agency Reuters quoted a senior US administration official as saying. Last year, US troops had left Afghanistan after about two decades in a decision that drew heavy criticism.

4. The operation had a legal basis, the official further added.

5. The operation was planned for weeks and finally given cleared by commander-in-chief Biden.