Hezbollah has said its top military commander Mustafa Badreddine died as a result of artillery shelling by a Sunni armed group in Damascus.
The Lebanese Shia group announced Badreddine's death on Friday and a military funeral was held for him on the same day in southern Beirut.
"Investigations have showed that the explosion, which targeted one of our bases near Damascus International Airport, and which led to the martyrdom of commander Mustafa Badreddine, was the result of artillery bombardment carried out by takfiri groups," Hezbollah said in a statement.
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"Takfiri" is a word used by the group to refer to armed Sunni groups.
Hezbollah earlier said it was working to "define the nature of the explosion and its cause, and whether it was the result of an air strike, or missile [attack] or artillery".
Badreddine, 55, was one of the highest ranking officials in the group, and believed to be responsible for its operations in Syria, where thousands of its members are fighting alongside Syrian President Bashar al-Assad.
Al Jazeera's Zeina Khodr, reporting from Beirut, said Badreddine was the most senior Hezbollah commander to have been killed in Syria since the conflict began.
"Hezbollah has suffered heavy losses in Syria, with some sources estimating that at least 1,200 fighters have died since the group started its involvement in the war," she said.
"Hezbollah's military intervention in Syria caused a divide in Lebanon. Some say it was totally wrong as it exposed Lebanon to threats. However, Hezbollah sees this as an existential decision because the Syrian government provides a lifeline to the group."
Source: Al Jazeera and agencies