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Israel launched a series of intense airstrikes in southern Lebanon early on Sunday in what it said was a pre-emptive strike against the Hezbollah militant group, threatening to trigger a broader regionwide war that could torpedo efforts to forge a cease-fire in Gaza.
The army said Hezbollah was planning to launch a heavy barrage of rockets and missiles toward Israel.
The Iranian-backed group had been promising to retaliate for Israel's assassination of a top commander late last month.
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Air raid sirens were reported throughout northern Israel, and Israel's Ben-Gurion international airport began diverting incoming flights and delaying takeoffs.
Soon afterwards, Hezbollah announced it had launched an attack on Israel with a "large number of drones" as an initial response to the killing of Fouad Shukur, a top commander with the group, in a strike in Beirut's southern suburbs last month.
Hezbollah said the attack was targeting "a qualitative Israeli military target that will be announced later" as well as "targeting a number of enemy sites and barracks and Iron Dome platforms."
The attack came as Egypt hosts a new round of talks aimed at ending Israel's war against Hamas, now in its 11th month. Hezbollah has said it will halt the fighting if there is a cease-fire.
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In the US, a spokesman for the National Security Council, Sean Savett, said President Joe Biden was "closely monitoring events in Israel and Lebanon.
"At his direction, senior US officials have been communicating continuously with their Israeli counterparts," Savett said.
"We will keep supporting Israel's right to defend itself, and we will keep working for regional stability."
In recent weeks, diplomats from the US and European countries have made a flurry of visits to Israel and Lebanon in an attempt to tamp down the escalation that they fear could spiral into a regional war, potentially pulling in the U.S. and Iran.
Last week, Israel's defence minister said he was moving more troops toward the Lebanese border in anticipation of possible fighting with the Iranian-backed group.
Israel's military spokesman, Rear Adm. Daniel Hagari, said early on Sunday: "In a self-defence act to remove these threats, the (Israeli military) is striking terror targets in Lebanon, from which Hezbollah was planning to launch their attacks on Israeli civilians."
"We can see that Hezbollah is preparing to launch an extensive attack on Israel, while endangering the Lebanese civilians," he added, without providing details.
"We warn the civilians located in the areas where Hezbollah is operating to move out of harm's way immediately for their own safety," he added.
Lebanese media reported strikes in the country's south without immediately providing more details. Social media footage showed what appeared to be strikes in southern Lebanon.
Israeli media cited the Israel Airports Authority for news of the flight cancellations. Flight-tracking data showed at least two El Al flights swinging far south and diverting after the announcement.
Hagari said "dozens" of israeli warplanes were striking targets in southern lebanon. He said air defenses, warships and warplanes were defending Israel's skies and involved in the operation
Hezbollah began attacking Israel almost immediately after the war with Hamas erupted on Oct. 7 with a Hamas cross-border attack.
Israel and Hezbollah have been exchanging fire nearly daily, displacing tens of thousands of people on both sides of the border and raising fears that the fighting could escalate into all-out war.
But until Sunday, both sides have been careful to avoid a broader conflagration.
Hezbollah is considered much more powerful than its ally, Hamas, with an estimated arsenal of arsenal of 150,000 rockets and missiles, including precision-guided missiles. In recent months the group has also stepped up its use of drones, against which Israel is less well-equipped to defend.
Israeli Prime Minister Benjamin Netanyahu's office said he and his defence minister, Yoav Gallant, were managing the latest operation from military headquarters in Tel Aviv. Gallant declared a "special situation on the home front," and Netanyahu's Security Cabinet was set to meet later on Sunday morning.
