Hezbollah was in turmoil Saturday after Israel took out two leaders of its elite operations unit in a strike on a command meeting in a Beirut suburb that Lebanese authorities said killed 31 people.
Lebanese Health Minister Firass Abiad said three children were among the dead in Friday's strike on an underground meeting room, which AFP journalists said left a huge crater in a densely populated neighbourhood of the capital's southern suburbs.
Israel said the strike killed the head of Hezbollah's elite Radwan Force, Ibrahim Aqil, and several other commanders.
The military said Saturday it was again hitting targets "belonging to the Hezbollah terrorist organisation in Lebanon", without elaborating.
Abiad said emergency services worked "through the night" to recover dead and wounded from the Beirut strike, adding that "a residential building collapsed on top of occupants" after the Israeli attack.
The Radwan Force has spearheaded Hezbollah's ground operations, and Israel has repeatedly demanded through international mediators that its fighters be pushed back from the border.
Coming hot on the heels of deadly sabotage attacks on communications devices, Friday's strike raised new questions about the Lebanese group's security arrangements and dealt a heavy blow to its fighters' morale.
Hezbollah said a second senior commander, Ahmed Mahmoud Wahbi, was also killed.
He headed the group's operations against Israel from the onset of the Gaza war in October until the start of this year.
Confirming the death of Aqil, who was wanted by the United States for involvement in the 1983 bombing of the US embassy in Beirut, Hezbollah hailed him as "one of its great leaders".
It was the second Israeli strike on the Hezbollah military leadership since the Gaza war began.
In July, an Israeli strike on Beirut killed Fuad Shukr, a top operations chief.
It also followed sabotage attacks on pagers and two-way radios used by Hezbollah on Tuesday and Wednesday, which killed 37 people and raised fears of a wider war.
Stéphane Dujarric, spokesman for UN Secretary-General António Guterres, said the world body was "very concerned about the heightened escalation" and called for "maximum restraint" from all sides.
Israel's military claimed it conducted a "targeted strike" against Aqil, which a source close to Hezbollah said killed a total of 16 Radwan Force members.
"The command of the Radwan Force was meeting in the basement of the building," the source said.
AFP journalists said the strike gutted the lower floors of the tower block.
Washington had offered a $7 million reward for information on Aqil, describing him as a "principal member" of an organisation that claimed the 1983 US embassy bombing which killed 63.
Israeli troops and Hezbollah fighters have battled each other along the Israel-Lebanon border since the Gaza war began.
The focus of Israel's firepower for nearly a year has been on Gaza, but that focus has now moved north.
Defence Minister Yoav Gallant said Israel's "enemies" would find no refuge, not even in Beirut's southern suburbs.
Military spokesman Daniel Hagari said Israel was "not aiming for a broad escalation in the region".
But Hamas called it a "brutal and terrorist aggression" and an "escalation".
Iran's foreign ministry accused Israel of seeking to "broaden the geography of the war".
Months of near-daily cross-border exchanges have killed hundreds in Lebanon, mostly fighters, and dozens in Israel and the occupied Golan Heights, forcing tens of thousands on both sides to flee their homes.
The latest blow to Hezbollah came after thousands of pagers and walkie-talkies exploded over two days.
Hezbollah chief Hassan Nasrallah vowed Thursday that Israel would face retribution for those blasts.
Before Friday's Beirut strike, Israel said Hezbollah had fired dozens of rockets from Lebanon following air attacks that destroyed dozens of the militant group's launchers.
Gallant told troops on Wednesday that "Hezbollah will pay an increasing price" as Israel tries to "ensure the safe return" of its citizens to border areas.
"We are at the start of a new phase in the war," he said.
Israeli Prime Minister Benjamin Netanyahu delayed by a day his scheduled departure to the United States, where he is due to address the UN General Assembly.
On Friday the UN's High Commissioner for Human Rights, Volker Türk, told the Security Council the attack on Hezbollah communications devices violated international law and could constitute a war crime.
The pagers and walkie-talkies exploded as their users were shopping in supermarkets, walking on streets, and attending funerals, plunging Lebanon into panic.
"I am appalled by the breadth and impact of the attacks," said Turk, adding that it "is a war crime to commit violence intended to spread terror among civilians".
International mediators, including the United States, have been scrambling to stop the Gaza war from becoming an all-out regional conflict.
In Gaza on Saturday, the civil defence agency said an Israeli strike on a school-turned-shelter in Gaza City killed 19 people – six women and 13 children.
Israel's war on Gaza has killed at least 41,391 people, according to the Palestinian enclave's health ministry.