Top Hezbollah commander Mustafa Badreddine has been killed in an attack in Syria, the Lebanese group has said.
It is the biggest blow to the Iranian-backed organisation since its military chief was killed in 2008.
Badreddine, 55, was one of the highest ranking officials in the group, and assessed by the US government to be responsible for Hezbollah's military operations in Syria, where it is fighting alongside the Syrian President Bashar al-Assad.
Hezbollah said Badreddine had been killed in a big explosion targeting one of its bases near Damascus airport.
It said an investigation was under way into whether it was caused by an airstrike, a missile attack, or artillery bombardment.
Lebanese TV station al-Mayadeen earlier reported he had been killed in an Israeli attack.
There was no immediate response from Israel which has struck Hezbollah targets inside Syria several times during the country's five-year conflict.
"We decline to comment," an Israeli military spokeswoman said.
A US Department of the Treasury statement detailing sanctions against Badreddine last year said he was assessed to be responsible for the group's military operations in Syria since 2011, and he had accompanied Hezbollah leader Sayyed Hassan Nasrallah during strategic coordination meetings with Assad in Damascus.
Badreddine, a brother-in-law of the late Hezbollah military commander, Imad Moughniyah, was one of five Hezbollah members indicted by the UN-backed Special Tribunal for Lebanon in the 2005 killing of statesman Rafik al-Hariri.
He was sentenced to death in Kuwait for his role in bomb attacks there in 1983.
He escaped from prison in Kuwait after Iraq, under the leadership of Saddam Hussein, invaded the country in 1990.
For years, Badreddine masterminded military operations against Israel from Lebanon and overseas and managed to escape capture by Arab and Western governments by operating clandestinely.
The US Treasury statement also said he had led Hezbollah ground offensives in the Syrian town of al-Qusayr in February 2013, a critical battle in the war when Hezbollah fighters defeated Syrian rebels in an area near the Syrian-Lebanese border.