An Israeli strike killed one of Hezbollah's senior field commanders and three of its fighters late on Tuesday, three security sources said, in a heavy blow to the Lebanese group which has been locked in hostilities with Israel for eight months.
In an apparent response, Hezbollah fired a barrage of around 50 launches from southern Lebanon into the Israeli-occupied Golan Heights, Israel said.
Hezbollah and Israel have been trading fire since the eruption of the Gaza war in October, with hostilities steadily escalating as both sides signal a readiness for a bigger confrontation.
Hezbollah confirmed the death of the commander, naming him as Taleb Abdallah, also known as Abu Taleb. He was killed in a strike in the village of Jouya in south Lebanon. His funeral is due to be held later on Wednesday.
He was the most senior Hezbollah commander killed during the hostilities, one of the sources said. The sources said he was Hezbollah's commander for the central region of the southern border strip.
There was no immediate comment from the Israeli military.
The security sources said the four members of the Iran-backed Hezbollah were likely targeted during a meeting.
The sources, who spoke on condition of anonymity, said the commander was senior to Wissam Tawil, a high-level Hezbollah commander killed in an Israeli strike in January.
Israeli strikes have killed some 300 Hezbollah fighters in Lebanon - more than it lost in 2006, according to a Reuters tally which puts the number of civilians killed at around 80. Attacks from Lebanon have killed 18 Israeli soldiers and 10 civilians, Israel says.
The Israeli military says it has killed more than 320 Hezbollah members, including at least 100 targeted after field operatives gathered "precise high-quality intelligence" on them.
Reporting by Laila Bassam in Beirut and James Mackenzie in Jerusalem