Saeed al-Shihri, described as the second-in-command of al-Qaeda in the Arabian Peninsula (AQAP), has been killed in a military operation, Yemeni government officials said.
Yemen's army website said on Monday that Shihri, a Saudi national, was killed, along with at least five other fighters, in a military operation in the remote Hadramawt province in eastern Yemen.
"The Saudi terrorist Saeed al-Shihri, the second man in al-Qaeda, was killed in a quality operation by the armed forces in Hadramawt," the 26sep.net news website reported.
"Other terrorist elements accompanying him were also killed," the army site added quoting what it said was a "high-ranking source", without mentioning when the operation took place.
Senior Yemeni defence ministry officials meanwhile told The Associated Press that Shihri was killed in an airstrike. They said a missile was believed to have been fired by a US-operated drone.
AQAP is described by Washington as the most dangerous and deadliest wing of al-Qaeda. The US has used unmanned drones to target the group, which has planned attacks on international targets including airliners.
A Yemeni security source told Reuters news agency that Shihri was killed in an operation last Wednesday which was thought to have been carried out by a US drone, rather than the Yemeni military.
The source said another Saudi and an Iraqi national were among the others killed.
Two senior US officials confirmed al-Shihri's death to the Associated Press, but could not confirm any US involvement in the airstrike.
US State Department spokesperson Victoria Nuland said she could not confirm Shihri's death.
"The US government... says that they are unable to confirm that Saeed al-Shihri, the reputed number two of al-Qaeda in the Arabian Peninsula, indeed has been killed," Al Jazeera's Rosiland Jordan reported from Washington, DC on Monday.
"There have been conflicting reports about whether a US-made or operated drone was the device that led to Shihri's death. There were also some reports and some claims of responsibility coming from Yemen itself.
"So there is a lot of confusion," she said.
The US does not usually comment on such attacks although it has used drones in the past to go after al-Qaeda members in Yemen.
Shihri is a former inmate of the US detention facility at Guantanamo Bay who was released to Saudi Arabia in 2007 and put through a Saudi rehabilitation programme for fighters.
After being released from Guantanamo in 2007, Shihri was put through a rehabilitation programme for violent religious extremists in Saudi Arabia. Soon after his release from there, he moved to Yemen and became deputy to Nasser al-Wahishi, the leader of AQAP.
Al-Wahishi is a Yemeni who once served as Osama bin Laden's personal aide in Afghanistan. Al-Qaida in Yemen has been linked to several attempted attacks on US targets, including the foiled Christmas Day 2009 bombing of an airliner over Detroit and explosives-laden parcels intercepted aboard cargo flights last year.
Yemen's government is trying to re-establish order after an uprising pushed out veteran ruler Ali Abdullah Saleh in February, but faces threats from armed fighters, southern secessionists and a Shia rebel movement in the north.
The protests and factional fighting have allowed al-Qaeda's regional wing to seize swathes of south Yemen, and Shia Muslim Houthi rebels to carve out their own domain in the north.
The lawlessness has alarmed the US and Yemen's much bigger neighbour Saudi Arabia, the top world oil exporter, which view the impoverished state as a new front line in their war on al-Qaeda and its affiliates.
Washington, which has pursued a campaign of assassination by drone and missile against suspected al-Qaeda members, backed a military offensive in May to recapture areas of Abyan province.
But fighters have struck back with a series of bombings and assassinations.