Israel launched a wave of air strikes across southern Lebanon early on Sunday in what it said was a pre-emptive strike on Hezbollah.
It came 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 ceasefire in Gaza, where Israel has been at war with the Palestinian group Hamas, an ally of Hezbollah, for more than 10 months.
By mid-morning, it appeared that the exchange had ended, with both sides saying they had confined their attacks to military targets.
But the situation remained tense, and the full extent of casualties and damage was not immediately known. At least three people were killed in the strikes on Lebanon.
The Israeli military said Hezbollah was planning to launch a heavy barrage of rockets and missiles towards Israel.
Soon afterwards, 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 air strike in Beirut last month.
The attacks came as Egypt is hosting a new round of talks aimed at ending the Israel-Hamas war.
Hezbollah has said it will halt the fighting if there is a ceasefire in Gaza. Iran supports both groups as well as militants in Syria, Iraq and Yemen who might join any larger conflict.
Israeli Prime Minister Benjamin Netanyahu, speaking at the start of a cabinet meeting, said the military had eliminated “thousands of rockets that were aimed at northern Israel” and urged citizens to adhere to directives from the Home Front Command.
“We are determined to do everything to defend our country, to return the residents of the north securely to their homes and to continue upholding a simple rule: Whoever harms us – we will harm them,” he said.
Air raid sirens were reported throughout northern Israel, and Israel’s Ben-Gurion airport closed and diverted flights for approximately an hour due to the threat of attack.
Israel’s Home Front Command has raised the alert level in northern Israel and encouraged people to stay near bomb shelters.
Israeli military spokesman Lieutenant Colonel Nadav Shoshani said Hezbollah had intended to hit targets in northern and central Israel.
He said initial assessments found “very little damage” in Israel, but that the military remained on high alert. He added that around 100 Israeli aircraft took part in Sunday’s strikes.
Lebanon’s Health Ministry reported that two people were killed and another two injured in the strikes in southern Lebanon.
Separately, a fighter for the Amal group, which is allied with Hezbollah, was killed in a strike on a car, Amal said.
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. But a later statement said “military operations for today have been completed”.
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.
It also dismissed Israel’s claim that its pre-emptive strikes had succeeded in warding off a stronger Hezbollah attack. Hezbollah did not provide evidence for its claims.
Randa Slim, a senior fellow at the Washington DC-based Middle East Institute, said Sunday morning’s exchange was “still within the rules of engagement and unlikely at this point to lead to an all-out war”.
In the US, a spokesman for the National Security Council, Sean Savett, said President Joe Biden is “closely monitoring events in Israel and Lebanon”.
“We will keep supporting Israel’s right to defend itself, and we will keep working for regional stability,” he added.
The Pentagon said Defence Secretary Lloyd Austin spoke with his Israeli counterpart, Yoav Gallant, about Israel’s defences against Hezbollah.
Austin “reaffirmed the United States’ ironclad commitment to Israel’s defence against any attacks by Iran and its regional partners and proxies,” a statement said.
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.
The US and other mediators see a ceasefire in Gaza as key to heading off a wider Middle East conflagration. Egypt planned to host high-level talks in Cairo on Sunday aimed at bridging the gaps in an evolving proposal for a truce and the release of Hamas hostages.
By Josef Federman and Abby Sewell, Associated Press