The Israeli military announced early Sunday that it was conducting "preemptive" strikes in Lebanon after detecting preparations for "large-scale" attacks by the Iran-backed militant group Hezbollah.
The military also warned Israelis to expect incoming missiles and drones launched by Hezbollah, with air alert sirens sounding across the country's north.
Hezbollah issued a statement shortly after announcing a large-scale operation.
The Middle East has been on edge for weeks after Hezbollah and Iran vowed to respond to an Israeli strike in Beirut that killed a senior commander of the group as well as the assassination in Tehran of Hamas's political leader, also blamed on Israel.
In a message addressed to the residents of southern Lebanon early Sunday morning, the Israeli military said: "We are monitoring Hezbollah's preparations to carry out large-scale attacks on Israeli territory near your homes. You are in danger. We are attacking and eliminating Hezbollah threats.
"Anyone who is near areas where Hezbollah operates should leave immediately to protect themselves and their families," the message posted on Telegram said.
In a separate statement released shortly before 5:00 am (0100 GMT), the military said its jets were striking targets in Lebanon "that posed an imminent threat" to Israelis.
Announcing the strikes in a video released at the same time, military spokesman Daniel Hagari said that "Hezbollah will soon fire rockets, and possibly missiles and UAVs, towards Israeli territory".
"From right next to the homes of Lebanese civilians in the south of Lebanon, we can see that Hezbollah is preparing to launch an extensive attack on Israel, while endangering the Lebanese civilians" he added, urging civilians there "to move out of harm's way".
"Hezbollah's ongoing aggression risks dragging the people of Lebanon, the people of Israel, and the whole region into a wider escalation."
Prime Minister Prime Minister Benjamin Netanyahu would hold a security cabinet meeting at 0400 GMT, his office said.
Israel's Ben Gurion International Airport announced that flights were being delayed and diverted on Sunday morning, while the country's emergency service group said was raising its level of alert.
Israel and Hezbollah have traded near-daily cross-border fire since the outbreak of the war in Gaza, triggered by Hamas's unprecedented October 7 attack on southern Israel last year.
Fears that the deadly exchanges could escalate into a full-scale conflict in Lebanon have only grown since Hezbollah's chief said his group was "obliged to respond" to Israel "whatever the consequences" after the Beirut strike last month that killed its military commander Fuad Shukr.
Hezbollah will retaliate "alone or in the context of a unified response from all the axis" of Iran-backed groups in the region, he added.
Shukr's killing, and that of Hamas political leader Ismail Haniyeh in Beirut hours later -- prompting fury from Hamas backer Iran as well as Hezbollah -- sent fears of a wider war soaring.
Hamas's October 7 attack on southern Israel resulted in the deaths of 1,199 people, most of them civilians, according to an AFP tally based on Israeli official figures.
Israel's retaliatory military campaign has killed 40,334 Palestinians in Gaza, according to Gaza's health ministry, which does not give details of civilian and militant deaths.
The UN rights office says most of the dead are women and children.
