JERUSALEM — Israeli fighter jets struck parts of south Lebanon and the Gaza Strip early Friday, in response to an unusually heavy rocket barrage from Lebanon on Thursday that the Israeli military blamed on Gaza-based Palestinian militias with branches on Lebanese territory.
The violence was considered the most serious along the Israel-Lebanon border since 2006, when Israel fought a war against Hezbollah, an Iran-backed Lebanese militia, and raised the risk of a wider multi-front conflagration between Israel, Palestinian militias and their allies.
The rocket attack on Thursday from Lebanon appeared to be in response to an Israeli police raid early on Wednesday on a mosque at a sensitive holy site in Jerusalem that had prompted widespread anger among Palestinians.
The Israeli military attributed the rocket fire to branches of Hamas and Palestinian Islamic Jihad, two militias based in Gaza that also have a presence in Lebanon, and which had condemned the mosque raid. The military said it believed that the militias had acted with the knowledge of Hezbollah, which dominates southern Lebanon.
Israeli warplanes retaliated before 1 a.m. on Friday with strikes on several sites in Gaza, most of them connected to Hamas’s military wing. Roughly four hours later, Israeli planes struck what the Israeli military said were three sites controlled by Hamas in south Lebanon, close to where the rocket barrage originated on Thursday afternoon.
“Israel’s reaction, tonight and in the future, will exact a significant price from our enemies,” Benjamin Netanyahu, the Israeli prime minister, said in a statement after the Israeli warplanes hit Gaza.
The Israeli response did not immediately prompt more rocket fire from Lebanon, but it did lead armed groups in Gaza to launch 44 short-range rockets toward Israel, according to the Israeli military. Most were intercepted by Israeli air defense systems or landed in open areas and only one hit a building, as both sides avoided causing the kind of damage that would turn the confrontation into all-out war.
Hamas said in a statement that Israel would “pay the price” for the police raid in Jerusalem and for “its aggression against our people in the Gaza Strip.” Nevertheless, militias in Gaza avoided firing rockets deep into Israeli territory — a sign that they did not seek a more drawn-out conflict. Israel also avoided targeting dense urban areas, though one strike caused collateral damage to a hospital, according to the Health Ministry in Gaza. No injuries were reported on either side.
A U.N. peacekeeping mission on the Israel-Lebanon border, the United Nations Interim Force in Lebanon, or UNIFIL, said in a statement that it was in touch with authorities in both countries and that “both sides have said they do not want a war.”
The flare-up in hostilities came as Jews celebrated the holiday of Passover and Muslims were in the middle of the holy fasting month of Ramadan.
Military experts said the barrage from Lebanon on Thursday was the heaviest in northern Israel since 2006, when Israel and Hezbollah last fought a full-scale war and Israel invaded parts of southern Lebanon.
The barrage, and the expectation of rocket fire from Gaza or Lebanon, prompted municipal councils in Israel to open public bomb shelters.
The violence compounded an already volatile security situation in the region. It came against a backdrop of rising tensions in Jerusalem, unusually high violence in the occupied West Bank and divisions within the Israeli military over a contentious plan by the Israeli government to overhaul the country’s judiciary.
The stage for Thursday’s hostilities was set on Wednesday morning, when the Israeli police raided the Aqsa mosque compound, a deeply sensitive holy site in Jerusalem that is sacred to both Jews and Muslims and is known to Jews as the Temple Mount. Officers arrested more than 350 Palestinians.
The Israeli police said it had been an essential operation to prevent troublemakers intent on stopping Jewish visitors from entering the site later in the day. But critics saw it as an unjustifiable assault on Muslim worshipers during the holiest month in Islam, and the raid caused fury across the Middle East. A day later, it appeared to prompt the rocket attack from Lebanon.
At least 34 rockets were fired in that attack, of which 25 were intercepted by Israeli air-defense systems and six landed in Israeli territory, according to the Israeli military. Several hours later, Lebanese news media and Israeli officials reported a short, second burst over northeastern Israel.
None of the groups blamed for the rockets — Hamas, Hezbollah and Islamic Jihad, all which are backed by Iran — claimed responsibility for the barrage. A spokesman for Hamas declined to comment, a spokesman for Islamic Jihad did not reply to a request for comment, while a media outlet run by Hezbollah said the source of the rockets was unknown.
The Lebanese prime minister, Najib Mikati, whose government has only limited influence over southern Lebanon, condemned the rocket fire.
Hamas’s leader, Ismail Haniyeh, was in Lebanon on Thursday to meet with leaders from Hezbollah and Islamic Jihad, adding to speculation about the three groups’ involvement in the rocket fire.
The rocket attack and Israeli strikes on Gaza followed weeks of escalation along the Israel-Lebanon border.
In an unusually brazen operation last month, a man who officials say was likely linked to Hezbollah crossed illegally from Lebanon to Israel and planted a bomb beside an Israeli highway. The attack severely injured an Israeli citizen.
The barrage on Thursday from Lebanon took Israelis by surprise.
Israel has a long history of conflict with Lebanese groups, occupying southern Lebanon between 1982 and 2000, and briefly invading again during the 2006 war. Since 2006, however, while armed groups have occasionally fired rockets from Lebanon into Israel, they have been in much smaller bursts and usually away from towns and cities.
Two of the rockets fired Thursday landed in built-up areas, sending up large plumes of smoke. The reverberations and shrapnel shattered several windows, and a rocket hit a warehouse. Several people inside Israel were injured, including one person hit by shrapnel, according to an emergency medical group, Magen David Adom.
A U.N. peacekeeping force that operates along the border, the United Nations Interim Force In Lebanon, described the situation as “extremely serious” and said it was in touch with the authorities in both countries.
The Israeli military denied Lebanese reports that Israel had already responded with cross-border artillery fire.
Israel regularly strikes Hezbollah-linked targets in Syria, where Hezbollah forces participate in the Syrian war. But confrontations on the Israeli-Lebanese border had been kept in relative check over the past decade.
Fears of a larger battle grew in recent weeks after the roadside bombing.
Hezbollah’s leader, Hassan Nasrallah, recently said that he believed Israel was on the verge of collapse, referring to the domestic political crisis over the proposed judicial overhaul that has exacerbated longstanding divisions within Israeli society.
The judicial crisis led Mr. Netanyahu to fire his defense minister, Yoav Gallant — at least nominally — after the minister called for a halt to the overhaul because of the anger it had caused within parts of the armed forces, endangering Israel’s military readiness. But Mr. Netanyahu never sent Mr. Gallant a letter officially confirming his dismissal, meaning that he remains in the post.
“As we have always predicted, great Israel has fallen,” Mr. Nasrallah said in a speech last month. “There is no trust in the army, political leaders or military leaders.”
But Mr. Netanyahu said on Thursday night that Israelis remained united in the face of external enemies. “The internal argument inside Israel will not prevent us from acting against them in every place and at any required time,” he said.
People close to Hezbollah said the group had also been angered by the Israeli raid on the Aqsa mosque compound.
“Aqsa is a red line for all Muslims,” Kassem Kassir, a political analyst close to Hezbollah, said. “The rocket attack is a message to Israel that we won’t be silent about all this escalation.” And he offered a warning: “If the escalation continues, the 20 rockets might become 1,000 in the coming days.”
Reporting was contributed by Hiba Yazbek in Jerusalem; Hwaida Saad in Beirut, Lebanon; Gabby Sobelman in Rehovot, Israel; and Farnaz Fassihi in New York.