Al-Qaeda's reclusive emir Ayman al-Zawahiri, who played a key role in the 9/11 attacks and later formed the group's regional affiliate in the Indian subcontinent, has been killed in a US drone strike in Kabul, in the biggest blow to the global terror network since its founder Osama bin Laden was eliminated in Pakistan in 2011.

Zawahiri, who assumed the leadership of al-Qaeda after the death of bin Laden, was killed in a drone strike carried out by CIA on Saturday evening at a house in a posh locality in the Afghan capital where he was sheltering to reunite with his family, US President Joe Biden said on Monday, declaring that "justice has been delivered and this terrorist is no more."

The 71-year-old Egyptian surgeon, on the FBI's Most Wanted Terrorist list, had a USD 25 million bounty on his head, was bin Laden's second-in-command during 9/11 attacks and took over as the head of al-Qaeda after his killing. He remained a visible international symbol of the terror group, 11 years after the US killed bin Laden during a raid in Pakistan's garrison city of Abbottabad in May 2011.

"My fellow Americans, on Saturday, at my direction, the United States successfully concluded an airstrike in Kabul, Afghanistan, that killed the emir of al Qaeda, Ayman al-Zawahiri," President Biden said.

"I authorised a precision strike that would remove him from the battlefield, once and for all," Biden, still isolating due to a Covid-19 reinfection, said in a speech from a balcony of the White House.

According to officials, Zawahiri was on the balcony of a safe house in Kabul when the drone fired two Hellfire missiles at him. Other family members were present, but they were unharmed.

"He was deeply involved in the planning of 9/11, one of the most responsible for the attacks that murdered 2,977 people on American soil. For decades, he was the mastermind of attacks against Americans, said Biden, referring to the victims of the 2001 attacks in which hijackers crashed passenger jets into landmark buildings in New York and Washington.

"Now, justice has been delivered and this terrorist leader is no more. People around the world no longer need to fear the vicious and determined killer," Biden said.

"The US continues to demonstrate our resolve and our capacity to defend the American people against those who seek to do us harm. We make it clear again tonight, that 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," he said.

Biden said the precision strike targeting was the result of the "extraordinary persistence and skill" of the nation's intelligence community.

The strike comes one year after Biden ordered the withdrawal of US troops from Afghanistan, prompting Taliban forces to rapidly seize control of the war-torn nation.

US Secretary of State Antony Blinken said the world is a safer place following the death of Zawahiri as he accused the Taliban regime of grossly violating its commitments to the international community by "hosting and sheltering" the al-Qaeda chief in Kabul.

Blinken said the US will continue to act against those who threaten the country, its people and its allies.

Zawahiri comes from a distinguished Egyptian family, according to the New York Times.

His grandfather, Rabia'a al-Zawahiri, was an imam at the prestigious al-Azhar University in Cairo. His great-uncle, Abdel Rahman Azzam, was the first secretary of the Arab League.

He played central role in the attacks on US embassies in Kenya and Tanzania in 1998, and eventually helped to mastermind the deadliest terror attack on American soil, when hijackers turned US airliners into missiles.

In September 2014, Zawahiri had announced the creation of Al-Qaeda's regional affiliate - the Al-Qaeda in the Indian Subcontinent (AQIS), taking advantage of sanctuaries in Afghanistan, Pakistan and Bangladesh.

"A new branch of Al-Qaeda was established - Qaeda al-Jihad in the Indian subcontinent, seeking to raise the flag of jihad,...and return the Islamic rule across the Indian subcontinent," Zawahiri had said at the time.

The al-Qaeda's regional affiliate was led by Asim Umar - an Indian and former member of Harkat-ul-Jihad al-Islami. Umar was killed in a joint US-Afghan military raid in the Afghan Province of Helmand in September 2019.

In April, Zawahiri's 8.43-minute video clip was released by the terror group online in which he had praised a Karnataka college student for confronting a group of students opposing hijab in her college in early February.

However, the girl's father distanced himself from Zawahiri's comments, terming them as "wrong" and said he and his family were living peacefully in India.

Meanwhile, the Taliban condemned the drone attack that killed Zawahiri.

Taliban senior spokesperson Zabihullah Mujahid said in a statement that the attack occurred on a residential home in the Sherpur area of Kabul, a diplomatic neighbourhood where several Taliban commanders currently reside.

Such actions are a repetition of the failed experiences of the past 20 years and are against the interests of the US, Afghanistan, and the region, Mujahid was quoted as saying by Afghanistan's Khaama Press News Agency.

Zawahiri was constantly on the move once the US-led invasion of Afghanistan began after the September 11, 2001, attacks. At one point, he narrowly escaped a US onslaught in the rugged, mountainous Tora Bora region of Afghanistan.

He made his public debut as a Muslim militant when he was in prison for his involvement in the 1981 assassination of Egyptian President Anwar Sadat.

By that time, Zawahiri, a young doctor, was already a committed terrorist who conspired to overthrow the Egyptian government for years and sought to replace it with fundamentalist Islamic rule. He proudly endorsed Sadat's assassination after the Egyptian leader made peace with Israel.

He spent three years in prison after Sadat's assassination. After his release, he made his way to Pakistan, where he treated wounded mujahideen fighters who fought against the Soviet occupation of Afghanistan.

That was when he met bin Laden and found a common cause, the CNN reported.

"We are working with brother bin Laden," he said in announcing the merger of his terror group, Egyptian Islamic Jihad, with al-Qaeda in May 1998.

"We know him for more than 10 years now. We fought with him here in Afghanistan," he had said.

Together, the two terror leaders signed a fatwa, or declaration: "The judgment to kill and fight Americans and their allies, whether civilians or military, is an obligation for every Muslim."