Muhsin al-Fadhli was killed in a "kinetic strike" on July 8 while travelling in a vehicle near the northwestern Syrian town of Sarmada, said Pentagon spokesman, Jeff Davis.
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 travelled from Pakistan to Syria to plot attacks on the West.
The Kuwaiti-born militant was so trusted by the inner circle of late al-Qaeda supreme 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," claimed Davis, who heads the Defence 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.
Officials say Khorasan is part of al-Qaeda’s Syrian branch, al-Nusra Front, though experts and activists cast doubt on the distinction between the two groups.
The four-year conflict in Syria has killed more than 220,000 people.
At least 84 journalists have been killed since 2011 in Syria, according to the Committee to Protect Journalists, while others remain missing or have been released for ransom.
An unprecedented spate of kidnappings by the so-called Islamic State militants starting in summer 2013 has kept most journalists away, particularly since the group began killing foreign journalists and aid workers it holds, starting with American journalist James Foley in August last year.
