Announcing that a US drone strike had killed Al-Qaeda chief Ayman al-Zawahiri, a mastermind of the 9/11 attacks, US President Joe Biden said "justice has been delivered and this terrorist leader is no more".

In a televised address, the US president said he gave the final go-ahead for the strike that successfully targeted Zawahiri in the Afghan capital over the weekend. Biden said he hoped Zawahiri's death would bring "closure" to families of the 3,000 people killed in the US on 9/11.

The US president later tweeted from his official handle that the attack demonstrates "our resolve and our capacity to defend the American people".

"The United States continues to demonstrate our resolve and our capacity to defend the American people against those who seek to do us harm. Tonight we made clear: No matter how long it takes. No matter where you try to hide. We will find you."

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.