Israeli airstrikes in Lebanon killed at least 31 people Monday following rocket fire from Tehran-backed Hezbollah after the killing of Iran’s supreme leader.

Israel vowed to intensify attacks and make Hezbollah pay a “heavy price,” targeting Beirut’s southern suburbs and south Lebanon, areas under the group’s influence.

“Hezbollah chose the Iranian regime over the State of Lebanon and initiated an attack on our civilians... they will pay a heavy price,” said Rafi Milo, head of Israel’s Northern Command. “The strikes continue, their intensity will increase.”

Lebanon’s health ministry reported 20 dead in Beirut’s southern suburbs and 11 in the south, with at least 149 wounded. Strikes hit the top floors of buildings, triggering fires and mass evacuations as families fled on motorcycles and in cars.

Hezbollah announced around 3 a.m. (0100 GMT) it had targeted an Israeli army site south of Haifa “with a barrage of high-quality missiles and a swarm of drones” in retaliation for the death of Ayatollah Ali Khamenei, blaming Israel for his killing.

Israel said it “precisely struck” senior Hezbollah members in Beirut and the south and warned residents of roughly 50 towns and villages to evacuate at least 1,000 meters from their homes. Army spokeswoman Ella Waweya said the order applied to Hezbollah strongholds in southern and eastern Lebanon.

The Iranian Revolutionary Guard Corps’ Telegram channel said Hezbollah had “officially entered the war.” Israeli forces said several projectiles from Lebanon fell in open areas with no reported injuries.

Lebanese President Joseph Aoun warned attacks from Lebanese territory risked drawing the country into regional conflict. Prime Minister Nawaf Salam called Hezbollah’s rocket fire “irresponsible” and vowed to “stop the perpetrators and protect the Lebanese people.”

Israeli military chief Eyal Zamir said, “Hezbollah opened a campaign against Israel overnight, and is fully responsible for any escalation.”