The Iranian-backed Lebanese Hezbollah movement said Friday that Mustafa Badreddine, one of the group's top commanders, was killed in an explosion in Syria this week.
Hezbollah added that an initial investigation showed that Badreddine, 55, had been killed Tuesday night in a large explosion that targeted one of its bases near the Damascus airport.
"[The] investigation now is concentrating on whether the blast was caused by bombardment, a missile or an airstrike," Hezbollah said in a statement.
Badreddine is to be buried later on Friday, the movement added.
Lebanese media reported earlier that Badreddine had been killed in an Israeli air raid.
Badreddine was reportedly responsible since 2011 for Hezbollah's operations in Syria, where the group has been fighting alongside the forces of President Bashar al-Assad against rebels.
Badreddine and five other Hezbollah members were indicted by the UN-backed Special Tribunal for Lebanon over the 2005 assassination of former Lebanese prime minister Rafik Hariri.
He was also linked to the 1983 bombing of the US Marine barracks in Beirut that killed 241 personnel.
He had lived in Syria in recent years.
"I will not return from Syria unless I am a martyr, carrying the banner of victory," Hezbollah's broadcaster al-Manar quoted Badreddine as previously saying.
In 2012, the US Treasury imposed sanctions on him along with other Hezbollah leaders, citing the movement's "active support" to al-Assad's regime and its role in "terrorist activities."
Badreddine is the third leading Hezbollah commander to have been killed in Syria in the past two years.
Hezbollah is believed to have lost more than 1,200 fighters in the Syrian conflict, which started with peaceful pro-democracy protests in March 2011.