Hezbollah has launched hundreds of rockets and drones at Israel as Israel's military says it struck Lebanon with around 100 jets to thwart a bigger attack, in one of the biggest clashes in more than 10 months of border warfare.
Missiles were visible curling up through the dawn sky, dark vapour trails behind them, as an air raid siren sounded in Israel and a distant blast lit the horizon, while smoke rose over houses in Khiam in southern Lebanon.
Three deaths were confirmed in Lebanon and none in Israel, where damage appeared to be limited. Hezbollah indicated it was not planning further strikes yet. Israel's foreign minister said the country did not seek a full-scale war.
Any major escalation in the fighting, which began in parallel with the war in Gaza, risks morphing into a regional conflagration drawing in Hezbollah's backer Iran and Israel's main ally the United States.
Sunday's strikes came as negotiators were meeting in Cairo in a last-ditch effort to conclude a halt to the fighting in Gaza.
The Iran-backed Lebanese group said it had fired 320 Katyusha rockets towards Israel and hit 11 military targets in what it called the first phase of its retaliation for Israel's assassination of Fuad Shukr, a senior commander, last month.
Israel's military said it had foiled a much larger attack with pre-emptive airstrikes after assessing that Hezbollah was preparing to launch the barrage, using 100 jets to strike more than 40 Hezbollah launch sites in southern Lebanon.
The strikes destroyed thousands of launcher barrels, aimed mostly at northern Israel but also targeting some central areas, Israel's military said.
Hezbollah dismissed Israel's statement that the group's attack had been foiled with pre-emptive strikes, saying it had been able to launch its drones as planned and that the rest of its response to Shukr's killing would take "some time".
Expectations of an escalation 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.
Israel's full cabinet is meeting on Sunday afternoon. Defence Minister Yoav Gallant declared a state of emergency and Foreign Minister Israel Katz said Israel would respond to developments on the ground but did not seek a full-scale war.
"We are determined to do everything possible to defend our country, to return the residents of the north safely to their homes and to continue to uphold a simple rule: Whoever harms us - we harm him," Prime Minister Benjamin Netanyahu said in a statement.
Lebanon's caretaker prime minister Najib Mikati met cabinet ministers at a session of the national emergency committee. Hezbollah leader Sayyed Hassan Nasrallah will speak on television later on Sunday, the group said.
Flights to and from Ben Gurion airport in Tel Aviv were suspended for around 90 minutes.
In northern Israel, warning sirens sounded and multiple explosions were heard around several areas as Israel's Iron Dome aerial defence system shot down rockets coming from southern Lebanon.
A security source in Lebanon said at least 40 Israeli strikes had hit various towns in the country's south in one of the densest bombardments since hostilities began in October.
One of the strikes on the town of Khiam killed a fighter from the Hezbollah-allied Shi'ite group Amal, two security sources told Reuters. Amal later announced his death.
An Israeli strike on al-Tiri killed two other people according to a security source and a medical sources. It was not immediately clear if they were fighters or civilians.
President Joe Biden was following events closely, the White House said.
"At his direction, senior US officials have been communicating continuously with their Israeli counterparts. We will keep supporting Israel’s right to defend itself, and we will keep working for regional stability," National Security Council spokesperson Sean Savett said.
The UN peacekeeping force in Lebanon and the UN's special coordinator's office in the country called on all sides to cease fire, calling the developments "worrying".
Egypt, one of the mediators in Gaza ceasefire talks, warned against the dangers of a new war front opening in Lebanon.
Yemen's Iran-backed Houthi group congratulated its ally Hezbollah on what it called a "great and brave" attack on Israel.
