Israel launched a wave of airstrikes across southern Lebanon early Sunday in what it said was a preemptive strike on Hezbollah, as the Iran-backed 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 came amid mounting fears of an all-out war that could draw in the United States, Iran and its allies across the region.

The Israeli military said it struck because 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.

By mid-morning, it appeared that the exchange had ended, with both sides saying they had only aimed at military targets. At least three people were killed in the strikes in Lebanon, while there were no reports of casualties in Israel. The situation remained tense.

U.S. and Arab mediators have been trying to broker a cease-fire in the 10-month-old Israel-Hamas war in Gaza, hoping to head off a wider conflagration, and the attacks came as Egypt hosts a new round of talks. Hezbollah has said it will halt the fighting if there is a cease-fire in Gaza. Iran supports both groups as well as other militia groups in Syria, Iraq and Yemen who might join any larger conflict.

Israeli Prime Minister Benjamin Netanyahu, speaking at the start of a Cabinet meeting, said the military had eliminated "thousands of rockets that were aimed at northern Israel" and urged citizens to adhere to directives from the Home Front Command.

"We are determined to do everything to defend our country, to return the residents of the north securely to their homes and to continue upholding a simple rule: Whoever harms us – we will harm them," he said.

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 raised the alert level in northern Israel and encouraged people to stay near bomb shelters.

Lt. Col. Nadav Shoshani, an Israeli military spokesperson, 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.

Lebanon’s Health Ministry reported that two people were killed and another two wounded in the strikes in southern Lebanon. Separately, a fighter for the Amal group, which is allied with Hezbollah, was killed in a strike on a car, Amal said.

Hezbollah said its attack involved more than 320 Katyusha rockets aimed at multiple sites in Israel and a "large number" of drones. It said the operation was targeting "a qualitative Israeli military target that will be announced later" as well as "enemy sites and barracks and Iron Dome (missile defense) platforms."

Hezbollah later announced the end of what it said was the first stage of retaliatory strikes, which it said would allow it to launch more attacks deeper into Israel. But a later statement said, "Military operations for today have been completed."

The group said all the exploding drones it launched hit their targets, without saying how many. It listed 11 bases, barracks and military positions that it said it targeted in northern Israel and the Israeli-annexed Golan Heights. It also dismissed Israel's claim that its preemptive strikes had succeeded in warding off a stronger Hezbollah attack. Hezbollah did not provide evidence for its claims.

Randa Slim, a senior fellow at the Washington, D.C.-based Middle East Institute, said Sunday morning’s exchange was "still within the rules of engagement and unlikely at this point to lead to an all-out war."

In the U.S., a spokesperson for the National Security Council, Sean Savett, said President Joe Biden was "closely monitoring events in Israel and Lebanon."

"At his direction, senior U.S. officials have been communicating continuously with their Israeli counterparts," Savett added. "We will keep supporting Israel’s right to defend itself, and we will keep working for regional stability."

The Pentagon said Defense Secretary Lloyd Austin spoke with his Israeli counterpart, Yoav Gallant, about Israel's defenses. The chairperson of the Joint Chiefs of Staff, Gen. C.Q. Brown, is on a visit to the region that is expected to take him to Israel, Egypt and Jordan.

In recent weeks, diplomats from the U.S. and European countries have made a flurry of visits to Israel and Lebanon in an attempt to tamp down the escalation that they fear could spiral into a regional war.

Hezbollah began attacking Israel almost immediately after the start of the war in Gaza on Oct. 7. Israel and Hezbollah have been exchanging fire almost daily, displacing tens of thousands of people on both sides of the border. Until Sunday, both sides had been careful to avoid all-out war.

Hezbollah, which fought Israel to a stalemate in the summer of 2006, is believed to be far more powerful than it was during that conflict. The United States and Israel estimate it has some 150,000 rockets and is capable of hitting anywhere inside Israel. The group has also developed drones capable of evading Israel's defenses, as well as precision-guided munitions.

Israel has one of the world’s best militaries and an extensive multi-tiered missile defense system, and it is backed by a U.S.-led coalition that helped it shoot down hundreds of missiles and drones fired from Iran earlier this year. The U.S. military has been building up its forces across the region in recent weeks.

Israel has vowed a crushing response in the case of all-out war, one that would likely demolish critical civilian infrastructure, especially in south Beirut and southern Lebanon, where Hezbollah’s main strongholds are located. A war would likely displace hundreds of thousands of people on both sides.

Hezbollah is a close ally of Iran, which has also threatened to retaliate for the killing of a senior Hamas leader, Ismail Haniyeh, in an explosion in Tehran last month that was widely blamed on Israel.

The U.S. and other mediators see a cease-fire in Gaza as key to heading off a wider Mideast conflagration. Egypt is hosting high-level talks in Cairo on Sunday aimed at bridging the gaps in an evolving proposal for a truce and the release of scores of hostages held by Hamas.

The talks were to be attended by CIA director William Burns, Qatari Prime Minister Mohammed bin Abdulrahman Al Thani and Egypt’s chief intelligence official, Abbas Kamel, according to two Egyptian officials who spoke on condition of anonymity to discuss the sensitive negotiations. The head of Israel’s Mossad intelligence agency, David Barnea, is also expected to attend, they said.

Hamas sent a delegation to the Egyptian capital to be briefed by Egyptian and Qatari mediators but is not directly taking part in the negotiations.

Israel has continued its brutal offensive on the Gaza Strip following an attack by the Palestinian group Hamas last Oct. 7, despite a U.N. Security Council resolution calling for an immediate cease-fire.

The onslaught has resulted in over 40,200 Palestinian deaths, mostly women and children, and over 93,000 injuries, according to local health authorities.

An ongoing blockade of Gaza has led to severe shortages of food, clean water and medicine, leaving much of the region in ruins.

Israel faces accusations of genocide at the International Court of Justice (ICJ), which has ordered a halt to military operations in the southern city of Rafah, where over 1 million Palestinians had sought refuge before the area was invaded on May 6.