Israeli airstrikes against Hezbollah in Lebanon killed at least 356 people and injured more than 1,200 others on Monday, Lebanon’s Health Ministry said, in the deadliest day of Israeli attacks there since at least 2006, when Israel last fought a war with the Iranian-backed militant group.

Rear Adm. Daniel Hagari, the Israeli military spokesman, said that since Monday morning, Israeli fighter jets had struck over 1,300 targets that he said were affiliated with Hezbollah, particularly in southern Lebanon and the Bekaa Valley. Main roads to Beirut, the capital, were clogged with people fleeing to what they hoped would be the safety of the capital, witnesses said.

As Israeli warplanes raced through Lebanon’s skies, Hezbollah launched its own barrage at Israel. Air-raid sirens rang out repeatedly as roughly 165 rockets and other munitions crossed into Israeli territory, according to the Israeli military. Most of the attacks were intercepted by Israel’s antimissile defense system, and there were no reports of deaths or serious casualties.

Hezbollah has been firing rockets and drones at Israel since last October in support of its ally, Hamas, prompting Israeli counterattacks. But Israel has ratcheted up its assaults on Hezbollah dramatically over the past week, raising fears that the current fighting could escalate into a full-scale war involving ground troops.

Prime Minister Benjamin Netanyahu of Israel told the Israeli public to expect “complicated days.” Lebanese officials said women and children were among the casualties from the Israeli strikes on Monday, but did not say how many of the dead were Hezbollah militants.

On Monday evening, Israel conducted an airstrike in Beirut in an attempt to assassinate Ali Karaki, one of the members of Hezbollah’s top leadership, according to three current and former Israeli officials with knowledge of the operation, who spoke on condition of anonymity to discuss military decisions.

Over the past several months, Israel has assassinated many senior commanders in Hezbollah. On Friday, Israel’s air force killed Ibrahim Aqeel, another veteran leader, alongside several other senior commanders of the group’s elite commandoes, in a bombing on a residential building in Beirut that killed and wounded dozens.

The strikes on Monday were Israel’s latest attempt to break the group’s resolve, after clandestine operations last week that blew up Hezbollah’s wireless devices, killing 37 people and wounding thousands. But so far Israel has failed to force Hezbollah to back down.

The group’s deputy chief on Sunday pledged to continue attacking until Israel ended its military campaign against Hamas in Gaza.