A US air strike in Syria has killed a key figure in an al-Qaida offshoot, the defence department said on Tuesday.

Muhsin al-Fadhli was killed on 8 July while in a vehicle near Sarmada, in Syria, a Pentagon spokesman, Captain Jeff Davis, said.

Davis did not elaborate on whether al-Fadhli was killed by a drone or a piloted aircraft.

Al-Fadhli was a leader of the Khorasan group, a cadre of al-Qaida operatives who were sent from Pakistan to Syria to plot attacks on the west. Officials say it is embedded in the al Nusra front, Syria’s al-Qaida affiliate.

A 2002 photograph of Muhsin al-Fadhli, from the al-Watan Kuwaiti newspaper Photograph: AFP/Getty Images

Al-Fadhli, who was previously based in Iran, was the subject of a $7m reward by the state department for information leading to his capture or death. He had been falsely reported as having been killed in 2014.

Davis said he was “among the few trusted al-Qaida leaders that received advanced notification of the September 11, 2001, attacks”.

Al-Fadhli was also involved in the October 2002 attacks against US marines on Faylaka island in Kuwait, and on the French ship MV Limburg, Davis said.

“His death will degrade and disrupt ongoing external operations of al-Qaida against the United States and our allies and partners,” he said.

Officials have said the Khorasan militants were sent to Syria by the al-Qaida leader Ayman al-Zawahiri to recruit Europeans and Americans whose passports allow them to board a US-bound airliner with less scrutiny.

According to classified US intelligence assessments, the Khorasan militants have been working with bomb-makers from al-Qaida’s Yemen affiliate to test new ways to slip explosives past airport security.

Officials were worried they would provide these sophisticated explosives to their western recruits who could sneak them on to US-bound flights.

Because of intelligence about the collaboration among the Khorasan group, al-Qaida’s Yemeni bomb-makers and western extremists, the transportation security administration decided last July to ban uncharged mobile phones and laptops from flights to the US from Europe and the Middle East.

The Khorasan group remains a threat, US officials said. Its existence demonstrates that core al-Qaida in Pakistan can still threaten the west, despite the damage done to that organisation by years of drone missile strikes.

“A seasoned, knowledgable and dangerous terrorist who actively sought to harm the United States and its allies has been taken off the battlefield for good,” said Adam Schiff, the top Democrat on the house intelligence committee. He said al-Fadhli would not be easily replaced.

The US military has periodically targeted the group as part of its air campaign in Syria, beginning with eight strikes against Khorasan targets last September.

Among those who have survived the bombs is a French-born jihadist who fought in Afghanistan with a military prowess that is of great concern to US intelligence officials.

David Drugeon, who was born in the Brittany region and converted to Islam as a youth, spent time with al-Qaida in the tribal areas of Pakistan before travelling to Syria, French officials say.