An Israeli airstrike targeting Hezbollah commanders killed at least 37 people in a suburb of Beirut on Friday, according to authorities.
Rescuers in the Lebanese capital hunted on Saturday for people still missing in rubble after the strike, which killed a senior Hezbollah military commander.
Ibrahim Aqil's death was confirmed by Hezbollah after Israel claimed to have taken out several top figures.
Israel's army said it hit an underground gathering of Aqil and leaders of Hezbollah's elite Radwan forces, and had almost completely dismantled its military chain of command.
The attack levelled a multi-story residential building in the crowded suburb and damaged a nursery next door, a security source told Reuters.
Three children and seven women were among those killed, according to Lebanon's health ministry.
UN political affairs chief Rosemary DiCarlo warned the Security Council that the Middle East risks a conflict that could "dwarf" previous devastation.
Emergency teams rushed to the attack site, with at least one residential building collapsed and others damaged.
The strike follows a week of attacks on Hezbollah's communication devices, killing 39 and wounding thousands.
Hezbollah members closed streets in Beirut, reeling from another blow after the recent explosions of their pagers and walkie-talkies.
This marks the first strike on Beirut since July, when Hezbollah military chief Fuad Shukr was killed.
IDF spokesman Daniel Hagari said Aqil was "gathered underground" with other senior operatives "in the heart of the Dahiyah neighbourhood, hiding among Lebanese civilians".
Hagari claimed the group was planning a "Conquer the Galilee" attack to "infiltrate Israeli communities and murder innocent civilians".
The US had been hunting Aqil and offering rewards for information leading to his capture. He was wanted for his role in 1980s bombings that killed hundreds of Americans in Beirut.
Hezbollah confirmed Aqil's death, calling him one of its "great jihadist leaders".
The Iran-backed group was formed in the 1980s to oppose Israel's occupation of southern Lebanon during the civil war.
