Israel launched a wave of airstrikes across southern Lebanon early Sunday in what it said was a pre-emptive strike on Hezbollah, as the militant group said it had launched hundreds of rockets and drones to avenge the killing of one of its top commanders last month.

The heavy exchange of fire threatened to trigger an all-out war that could draw in the United States, Iran and militant groups across the region. It could also torpedo efforts to forge a cease-fire in Gaza, where Israel has been at war with the Palestinian group Hamas, an ally of Hezbollah, for over 10 months.

The Israeli military said Hezbollah was planning to launch a heavy barrage of rockets and missiles toward Israel. Soon after, Hezbollah announced it had launched an attack on Israeli military positions as an initial response to the killing of Fouad Shukur, one of its founders, in an Israeli airstrike in Beirut last month.

The attacks came as Egypt hosts a new round of talks aimed at ending the Israel-Hamas war. Hezbollah has said it will halt the fighting if there is a cease-fire in Gaza. Iran supports both groups as well as militants in Syria, Iraq and Yemen who might join any larger conflict.

Air raid sirens were reported throughout northern Israel, and Israel's Ben-Gurion international airport diverted incoming flights and delayed takeoffs for a time. Israel's Airports Authority said flights resumed at 7 am local time.

Rear Adm. Daniel Hagari, the top Israeli military spokesman, said “dozens” of warplanes were striking targets in southern Lebanon. He said air defences, warships and warplanes were defending Israel's skies and involved in the operation.

Hezbollah said its attack involved more than 320 Katyusha rockets aimed at multiple sites in Israel and a “large number” of drones. It said the operation was targeting “a qualitative Israeli military target that will be announced later” as well as “enemy sites and barracks and Iron Dome (missile defence) platforms.”

Hezbollah later announced the end of what it said was the first stage of retaliatory strikes, which it said would allow it to launch more attacks deeper into Israel. The group said all the exploding drones it launched hit their targets, without saying how many. It listed 11 bases, barracks, and military positions that it said it targeted in northern Israel and the Israeli-annexed Golan Heights.

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.

Last week, Israel's defence minister said he was moving more troops toward the Lebanese border in anticipation of possible fighting with Hezbollah.

Hagari, the military spokesman, said early Sunday that "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.

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 Sunday morning.

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.

Hezbollah began attacking Israel almost immediately after the start of the war in Gaza, which was triggered by Hamas' Oct. 7 attack into Israel. Israel and Hezbollah have been exchanging fire almost daily, displacing tens of thousands of people on both sides of the border. Until Sunday, both sides had been careful to avoid all-out war.

Hezbollah, which fought Israel to a stalemate in the summer of 2006, is believed to be far more powerful than it was during that conflict. The United States and Israel estimate it has some 150,000 rockets and is capable of hitting anywhere inside Israel. The group has also also developed drones capable of evading Israel's defences as well as precision-guided munitions.

Israel has one of the world's best militaries and an extensive multi-tiered missile defence system, and it is backed by a US-led coalition that helped it shoot down hundreds of missiles and drones fired from Iran earlier this year. The US military has been building up its forces across the region in recent weeks.

Israel has vowed a crushing response in the case of all-out war, one that would likely demolish critical civilian infrastructure, especially in south Beirut and southern Lebanon, where Hezbollah's main strongholds are located. A war would likely displace hundreds of thousands of people on both sides.

Hezbollah is a close ally of Iran, which has also threatened to retaliate for the killing of a senior Hamas leader, Ismail Haniyeh, in an explosion in Tehran last month that was widely blamed on Israel, which has not said whether it was involved.