Hezbollah launched hundreds of rockets and drones against Israel on Sunday in retaliation for the assassination of a senior commander in Beirut last month, the Iranian-backed movement said, as Israel's cabinet met to prepare a response.
Israeli jets hit targets in Lebanon shortly before the strikes as the military assessed that Hezbollah was preparing to begin the barrage, the military said.
Hezbollah said it had launched more than 320 Katyusha rockets towards Israel and hit 11 military targets.
It said the barrage had completed "the first phase" of its response to the assassination of Fuad Shukr, a senior commander, in Beirut but that the full response would take "some time".
Defence Minister Yoav Gallant said Israel would do whatever necessary to defend itself.
Gallant declared a state of emergency, and flights to and from Ben Gurion airport in Tel Aviv were suspended, but the airport authority said normal operations were expected to resume by 7 a.m.
A resident of the southern Lebanese town of Zibqeen, some 7 km (4 miles) from the border, told Reuters it was the first time he had awakened "to the sound of planes and the loud explosions of rockets - even before the dawn prayer. It felt like the apocalypse."
FEARS OF REGIONAL CONFLICT
Expectations of an escalation between the two sides had risen since a missile strike in the Israeli-occupied Golan Heights last month killed 12 youngsters and the Israeli military assassinated Shukr in Beirut in response. The escalation has drawn fears of a wider regional conflict, potentially involving both the United States and Iran. President Joe Biden was following events closely, the White House said.
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