Israeli air strikes targeting Hezbollah in Lebanon killed 274 people on Monday, the Lebanese health ministry said, in the deadliest day in Lebanon since the country's civil war which ended in 1990.
The dead included 21 children and 31 women while 1,024 were wounded, health minister Firass Abiad told a press conference.
The air attacks sent residents of south Lebanon fleeing for safety northward as some public schools were turned into shelters.
"Air Force fighter jets have attacked ... about 800 Hezbollah terrorist targets in southern Lebanon and in the Bekaa area deep in Lebanon in several waves of attacks throughout the day," the Israeli air force said in a post on X.
"The IDF attacked buildings where Hezbollah hid rockets, missiles, launchers, unmanned aerial vehicles and other military infrastructure," it said, adding that it aimed to destroy the group's "capabilities and infrastructure".
For its part, Hezbollah claimed responsibility for rocket attacks targeting Israeli military sites.
Israeli prime minister Benjamin Netanyahu on Monday warned of difficult days ahead and urged unity among Israelis.
"I promised that we would change the security balance, the balance of power in the north - that is exactly what we are doing," he said. "No country can accept the wanton rocketing of its cities."
Israel last week updated the aims of its nearly one-year-old war against Hamas in Gaza and Hezbollah in Lebanon to include returning tens of thousands of civilians evacuated from its northern border to their homes.
Starting on Tuesday, two back-to-back days of explosions hit communication devices held by Hezbollah fighters and on Friday an air strike killed senior members from the group while they were meeting in southern Beirut, in attacks which killed scores.
Hundreds of others, including Iran's ambassador to Lebanon, were left maimed and in hospital.
Iran's foreign ministry spokesman Nasser Kanaani condemned the killing "hundreds of defenceless people", in comments on the ministry's official Telegram channel.
"We strongly condemn the silence and support of the US and some Western states to the Zionist regime and warn against the continuation of the regime's crimes against Palestinians and its attempt to expand war to the entire region," Kanaani said. "There will be dangerous consequences to the Zionists' new adventure."
Major General Hossein Salami, the commander of Iran's Islamic Revolutionary Guard Corps, said last week that the so-called Axis of Resistance grouping of Islamist militias Tehran leads in the region would punish Israel for the attacks.
But an Israeli military official, speaking on condition of anonymity to the Associated Press, said the Jewish state had no immediate plans for a ground incursion and that the aerial bombardments were aimed at curbing Hezbollah's ability to fire rockets.
The United Nations has urged against a further escalation of hostilities as its special coordinator for Lebanon, Jeanine Hennis-Plasschaert touched down in Israel on Monday.
"There is no military solution that will make either side safer. With the wellbeing of civilians on both sides of the Blue Line and the stability of the region at stake, space must be given for diplomatic efforts to succeed," she said in a statement, referring to the Israel-Lebanon border.