Beirut: Hezbollah's top military commander Mustafa Badreddine has been killed in a blast at a base near Damascus airport, the group said on Friday, one of the biggest blows to its leadership the organisation has ever sustained.
There was no immediate claim of responsibility for the killing and Hezbollah said it was still investigating the cause of the explosion.
Hezbollah deputy leader Sheikh Naim Qassem said the result of the investigation into Mustafa Badreddine's killing would be announced no later than Saturday morning.
"We will announce in detail the cause of the explosion and the party responsible for it," he said, adding that there were clear indications of who was behind the attack and the method used.
Speaking at Badreddine's funeral, he also vowed that the group would continue on the "path" of Badreddine.
At least one Hezbollah figure blamed Israel, which has struck Hezbollah targets in Syria several times in the past since civil war started there in 2011. Israel declined to comment.
The US government believed Badreddine, 55, was in charge of Hezbollah's military operations in Syria.
He was a brother-in-law of Imad Moughniyah, Hezbollah's long serving military commander, who was killed by a bomb planted in his car in Damascus in 2008 that Hezbollah blamed on Israel.
At least four prominent figures in Hezbollah have been killed since January 2015.
Hezbollah said an investigation was underway into whether the explosion at the base was caused by an air strike, a missile attack or artillery bombardment. It did not say when he was killed.
"This is an open war and we should not preempt the investigation but certainly Israel is behind this," said Nawar Al Saheli, a Hezbollah member of Lebanon's parliament. "The resistance will carry out its duties at the appropriate time."
When asked by an interviewer on Israel Radio about possible Israeli involvement, cabinet minister Zeev Elkin, a confidant of Prime Minister Benjamin Netanyahu, declined to comment.
Hezbollah, a political and military movement, is Lebanon's most powerful group, having grown stronger since forcing Israel to end its 22-year occupation of southern Lebanon in 2000. The sides fought a 34-day war in 2006, their last major conflict.
"We don't know if Israel is responsible for this," Yaakov Amidror, a former national security adviser to Netanyahu, told Israel's Army Radio.
Announcing his death, Hezbollah quoted Badreddine as saying he would return from Syria victorious or as a martyr. A photo released by the group showed him before his death, smiling and wearing a camouflage baseball cap.
His death sparked wide condemnation from Lebanese political allies.
"His martyrdom is a big loss for the Lebanese in their fight against Israeli aggression and terrorism," Finance Minister Ali Hassan Khalil told Hezbollah's Al Manar TV in reference to Israel and miliant groups.
"His loss will leave a vacuum but the lesson is to continue on the path that he chose -- resistance until victory is achieved."