The Israeli military says its warplanes are hitting Hezbollah targets in Lebanon after detecting moves to fire missiles and rockets into Israel.
“In a self-defence act to remove these threats, the IDF (Israel Defense Forces) is striking terror targets,” IDF spokesman Daniel Hagari said.
Israel said Lebanese civilians had been warned to immediately leave areas where Hezbollah, an Iran-backed Shia Muslim group, was operating.
Shortly afterwards, Hezbollah said it had launched a large-scale drone attack on Israel in response to last month’s killing of the group’s senior military commander.
Across northern Israel sirens warning of incoming rockets were heard sounding early on Sunday.
There were no immediate reports of any injuries.
In a statement, the IDF said Hezbollah “has just launched over 150 projectiles from Lebanon toward Israeli territory”.
Meanwhile, Hezbollah said it had already launched more than 320 Katyusha rockets, saying that it was hitting 11 Israeli military bases and barracks.
Israeli Prime Minister Benjamin Netanyahu said he was convening an urgent meeting of his security cabinet.
Mr Netanyahu’s office said the prime minister and Defence Minister Yoav Gallant were “managing the situation” from the IDF’s military base in Tel Aviv.
Meanwhile, Hezbollah said in a statement that its drone attack on Israel was in response to “the brutal Zionist aggression, which led to the martyrdom of… Fuad Shukr”.
The senior military commander of the Shia group was killed in an Israeli air strike in Lebanon’s capital Beirut in July.
Israel has been exchanging fire with the Lebanon-based militant group since the start of the war last October with Hamas in the Gaza Strip.
Hamas, like Hezbollah, is backed by Iran.