Top Hezbollah commander Mustafa Badreddine has been killed in an Israeli air strike at the Lebanese-Israeli border, in the biggest blow to the group since the 2008 killing of its military commander.
Badreddine, 55, was one of the highest ranking officials in the group, according to security sources.
"He took part in most of the operations of the Islamic resistance since 1982," Hezbollah said in a statement announcing his death on Friday, describing Badreddine as "the great jihadi leader".
He was killed on Tuesday night, said the statement from the Lebanese Shi'ite group.
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.
Hezbollah, which is backed by Iran, is fighting in Syria in support of President Bashar al-Assad.
Badreddine, a brother-in-law of the late Hezbollah military commander, Imad Moughniyah, was 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 said Badreddine had accompanied Hezbollah leader Sayyed Hassan Nasrallah during strategic coordination meetings with Assad in Damascus.
It 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.
Around 1,200 Hezbollah fighters are estimated to have been killed in the Syrian conflict.