“There is a sense that he may no longer be with us,” said one senior US official familiar with intelligence reports about the strike. “If the reports are true, it would be a significant victory.”

The official said Masri, a top aide to No. 2 al-Qaeda commander Ayman al-Zawahiri, had run explosives-training camps in Afghanistan before the US-led invasion in 2001, and had more recently conducted training for al-Qaeda recruits in bases along the Afghan-Pakistani border. Masri, 54, whose given name was Midhat Mursi, had also been in charge of al-Qaeda’s rudimentary biological and chemical weapons programs before being driven from Afghanistan in 2001.

Although CIA officials declined to comment on the attack, CIA-operated Predator drones are known to have carried out several successful missile strikes on al-Qaeda and Taliban leaders inside Pakistan’s autonomous tribal region this year, including a January attack that killed Abu Laith al-Libi, a senior al-Qaeda commander.