Abdel Bari al-Atwan, editor-in-chief of al-Quds Al Arabi newspaper, said al-Masri "represented the old guard of al-Qaeda, so it is going to be very difficult to replace him".

"This was a huge success for the Americans in their pursuit of al-Qaeda leaders," he said.

"Al-Qaeda usually replace these people in this situations, but actually, they won't have the people with the determination and expertise of this man.

"He managed to experiment a lot with chemical and biological weapons in the Tora Bora area.

"Maybe he has disciples, but there's no one with his skills. He knows the ideology of al-Qaeda very well and I don't believe he will be easily replaced by other people."

Senior bomb maker

Al-Masri, a 55-year-old Egyptian chemist, was regarded as one of the group's senior bomb makers.

The statement said he had left behind him a generation of so-called students who would avenge his killing.

On Saturday, a Pakistani Taliban spokesman denied a US media report that Ayman al-Zawahri, al-Qaeda's deputy leader, had been wounded or killed in what was believed to be the same US missile strike that killed Masri.

Al Jazeera's Zeina Khodr, reporting from Kabul, said: "Afghanistan is seeing the worst violence since the Taliban was ousted in 2001.

"The killing of Masri also comes at a time when there have been increasing reports that a number of al-Qaeda fighters are now infiltrating into Afghanistan and working here alongside the Taliban."

Ahmed Rashid, the Pakistani journalist and author, told Al Jazeera: "Masri has left behind a new generation of people that he did train."

"It is quite possible that his death could spark retaliation outside the region ... inside the region, in Afghanistan and in Pakistan, we are seeing an all-out offensive."