Muhsin al-Fadhli was killed in a “kinetic strike” on July 8 while traveling in a vehicle near the northwestern Syrian town of Sarmada, said Captain Jeff Davis, a Pentagon spokesman.
He did not confirm whether a drone or a manned aircraft had killed Fadhli, 34.
Fadhli was allegedly the leader of the Khorasan Group, a group of senior Al-Qaeda members who have traveled from Central Asia and elsewhere in the Middle East to Syria to plot attacks on the West.
The Kuwaiti-born militant was so trusted by the inner circle of late Al-Qaeda leader Osama bin Laden that he was among the few who knew in advance about the September 11, 2001 attacks on the United States, according to US intelligence.
“His death will degrade and disrupt ongoing external operations of Al-Qaeda against the United States and its allies and partners,” said Davis, who heads the Defense Department’s press operations.
Counterterrorism expert Bruce Riedel, however, a former CIA analyst, called Fadhli's death a “serious but not fatal” blow to Al-Qaeda in Syria.
Davis said Fadhli was also involved in October 2002 attacks against US Marines on Kuwait's Failaka Island and on the MV Limburg, a French oil tanker.
He was reported to have been previously targeted in a US air strike in September, but his death was not confirmed by US officials at the time.