Al-Qaeda said its number three leader and Osama bin Laden's one-time top money man Mustafa Abu al-Yazid has been killed along with several family members, in what would be a major blow to the global terror network.
US monitoring groups said the death of Yazid, who was the leader of Al-Qaeda in Afghanistan and its liaison with the Taliban for three years, was announced by the group in a message to jihadist websites on Monday.
"We have strong reason to believe that's true, and that (Yazid) was killed recently in Pakistan's tribal areas," a US official said. "In terms of counterterrorism, this would be a big victory."
Yazid, one of a number of Egyptians in the higher echelons of Al-Qaeda, was a founding member of the network and a former treasurer to bin Laden who was accused of channelling money to some of the September 11 hijackers.
Yazid, also known as Sheikh Said al-Masri, would be one of the highest profile Al-Qaeda leaders killed since US President Barack Obama took office.
"Al-Masri was the group's chief operating officer, with a hand in everything from finances to operational planning," the US official said.
"He was also the organisation's prime conduit to bin Laden and (Al-Qaeda number two Ayman) Zawahiri," he said. "He was key to Al-Qaeda's command and control."
The Al-Qaeda message carried by the SITE group that monitors Islamist websites did not say where or when Yazid was killed other than to speak of his "martyrdom".
But it said his wife, three of his daughters, his granddaughter, and other men, women and children were killed along with him.
"His death will only be a severe curse by his life upon the infidels. The response is near," according to the message translated by SITE.
Some US media reports said Yazid was killed in a US drone strike in the Pakistani tribal areas on the Afghan border, where the United States has been waging a covert drone war against militants in areas outside direct government control.
In the most recent such attack, local officials said six militants were killed on May 22 in North Waziristan, a rugged area Washington labels a global headquarters of Al-Qaeda and the most dangerous place on Earth.
"Though these terrorists remain extremely dangerous and determined to strike at the United States, the removal from the battlefield of top leaders like al-Masri is further proof that the tribal areas are not quite the safe haven Al-Qaeda and its allies thought them to be," the US official said.
Waheed Omar, spokesman for Afghan President Hamid Karzai, said the death of any "terrorist involved in terrorism activities in either Afghanistan or Pakistan, or on the borders, is a development (that) advances the cause".
But he added: "We are not sure of any significant impact of the killing of one of the members of Al-Qaeda.
Analysts said several of Yazid's predecessors as Al-Qaeda number three had been killed or captured, while there were rumours in 2008 that he himself had been killed.
Yazid, 54, was among those whose assets were frozen by the US Treasury in the wake of the September 11, 2001 attacks on the United States.
According to the FBI, it was Yazid who transferred funds via Dubai for Mohammed Atta, Marwan al-Shehhi and Wal al-Shehri, three of the hijackers who flew aircraft into the Twin Towers and the Pentagon.
Yazid, wearing thick glasses and a white turban, appeared in a number of videos released by Al-Qaeda since he first emerged as head of Al-Qaeda in Afghanistan in May 2007.
A former member of the Islamic Jihad movement in Egypt, he had close links with fellow Egyptian Zawahiri and served time in jail over the 1981 assassination of Egyptian president Anwar Sadat.
"This is one of the most significant blows against Al-Qaeda in recent years and its impact will be felt by the group," said Ben Venzke of intelligence analysis group IntelCenter.
He said Yazid ran Al-Qaeda operations in Afghanistan and Pakistan.
According to Yasser al-Sirri, the director of the London-based Islamic Observatory, Yazid was born in December 1955 in Egypt.
Sirri said at the time of Yazid's appearance in May 2007 that he was "trusted by bin Laden, for whom he ran businesses in Sudan" when the founder of Al-Qaeda lived in exile there before Khartoum expelled him in 1996.
"Yazid is known for his integrity and management skills, but has never taken organisational or military responsibility at the heart of Al-Qaeda, of which he was one of the founders in 1989," Sirri said.
Yazid's last public statement was on May 4, when he delivered eulogies for the two top Al-Qaeda leaders in Iraq who were killed in April.