Israel launched a wave of airstrikes across southern Lebanon early Sunday in what it said was a pre-emptive strike on Hezbollah, as the militant group said it had launched hundreds of rockets and drones to avenge the killing of one of its top commanders last month. The heavy exchange of fire threatened to trigger an all-out war that could draw in the United States, Iran and militant groups across the region. It could also torpedo efforts to forge a cease-fire in Gaza, where Israel has been at war with the Palestinian group Hamas, an ally of Hezbollah, for over 10 months. The Israeli military said Hezbollah was planning to launch a heavy barrage of rockets and missiles toward Israel. Soon after, Hezbollah announced it had launched an attack on Israeli military positions as an initial response to the killing of Fouad Shukur, one of its founders, in an Israeli airstrike in Beirut last month. Air raid sirens were reported throughout northern Israel, and Israel's Ben-Gurion international airport closed and diverted flights for approximately an hour due to the threat of attack. Israel’s Home Front Command has raised the alert level in northern Israel and encouraged people to stay near bomb shelters. Lt. Col. Nadav Shoshani, an Israeli military spokesman, said Hezbollah had intended to hit targets in northern and central Israel. He said initial assessments found “very little damage” in Israel, but that the military remained on high alert. He said around 100 Israeli aircraft took part in Sunday’s strikes.