At least three of the dead were leaders in Islamic Jihad, the group said, and officials said at least 10 civilians, including children, were killed in the strikes, which Israel said had hit the planners of attacks.
The Israeli military said on Tuesday that it had conducted airstrikes against leaders of the group Islamic Jihad in Gaza, in strikes that Palestinian officials said killed at least 10 civilians, including children. Both sides braced for a potentially sharp escalation in cross-border violence.
The predawn airstrikes hit residential buildings across the Palestinian coastal territory as people slept, roughly a week after an exchange of fire between Islamic Jihad and Israel. The group confirmed that three of its senior leaders were among the dead.
The Palestinian Ministry of Health in Gaza said at least 13 people were killed, and at least 20 were injured. Among the dead were Dr. Jamal Khiswan, a director of the Wafa Hospital in Gaza, and his wife and son.
The Israeli military said it had targeted and killed Khalil Bahitini, who it said had been responsible for launching rockets against Israel over the past month; Tareq Ezzedine, who had directed attacks against Israelis in the occupied West Bank; and Jihad Al-Ghanam, another high-ranking leader of the group.
Israel also attacked Islamic Jihad military sites and infrastructure.
The military wing of Islamic Jihad — which Israel, the United States and many other Western countries classify as a terrorist organization — said in a statement that the three leaders had been killed “as a result of a cowardly Zionist assassination at dawn today.”
The group said that some of the wives and children of the men had also been killed, adding that “the blood of the martyrs will increase our resolve, and we will not leave our positions and the resistance will go on, God willing.”
Schools and universities across Gaza canceled classes and exams as search and rescue crews were still digging through the rubble Tuesday morning, and the Gazan government ordered Palestinian fishing vessels not to go out to sea. The Gaza Strip operates under a severe land, air and sea blockade by Israel and Egypt.
Gaza is dominated by Hamas, a larger Islamic militant group that sometimes acts in coordination with Islamic Jihad and, at other times, acts to restrain it. The military wing of Hamas issued a statement mourning those killed in the Israeli campaign. The question of whether Hamas will join the Iranian-backed Islamic Jihad in any retaliatory action against Israel could determine the length and intensity of the round of fighting.
About five hours after Israel’s opening strikes, Lt. Col. Richard Hecht, a spokesman for the Israeli military, told reporters that the military had achieved its objectives and that it was largely “up to Hamas” to determine what would happen next.
Colonel Hecht said he was aware of the reports of civilian deaths, which he said would be “tragic,” but had no immediate further comment about them, except to say that Israel conducted a “pinpoint” operation involving 40 aircraft.
Amid the strikes, the Israeli military instructed residents of Israel living within a radius of 25 miles of the border of the Palestinian coastal territory to stay close to bomb shelters for the next two days, in apparent expectation of retaliatory rocket fire.
Israel’s minister of defense, Yoav Gallant, declared an emergency situation along the border. The military issued a map of road closings in the area and closed border crossings to people and goods. Col. Hecht said the ministry had instructed the army to be ready to call up reservists.
The deadly airstrikes started at about 2 a.m. Tuesday and initially hit Gaza City and the southern city of Rafah, along the border with Egypt. Two hours later, the military said it was striking additional targets of Islamic Jihad, including what it described as weapons manufacturing sites and military compounds.
The operation, which the military code-named “Shield and Arrow,” follows a short burst of violence after the death last week of a prominent Palestinian prisoner, Khader Adnan, who had been on a hunger strike for 87 days to protest his detention.
Islamic Jihad fired more than 100 rockets and mortar shells toward southern Israel in the 24 hours after his death. One barrage fired in the middle of the day severely wounded a Chinese construction worker in the Israeli border town of Sderot.
Another brief flare-up a month ago, during the Muslim holy month of Ramadan, took place after an Israeli police raid in Jerusalem on the Aqsa mosque compound, a revered site known to Jews as Temple Mount. That prompted the Palestinian groups in Gaza as well as militias in Lebanon — led by Hamas, according to the Israeli military — to fire barrages of rockets at Israel.
Israel struck back at the militias in southern Lebanon, as well as at Hamas military sites in the Gaza Strip. It also hit militant sites in Gaza after last week’s rocket fire, killing a 58-year-old man, according to the Gaza health ministry.
But far-right members of the Israeli governing coalition complained that Israel’s response had been too weak, and the ultranationalist minister of national security, Itamar Ben-Gvir, demanded that Israel resume its policy of targeted assassinations of militant leaders.
Mr. Gallant, the defense minister, said in a Twitter post before dawn on Tuesday that the military and the Shin Bet, Israel’s domestic security agency, had “precisely carried out their mission against the leadership of the Islamic Jihad in the Gaza Strip. Any terrorist who harms the citizens of Israel will regret it. We will pursue and catch up with our enemies,” he added.
Iyad Abuheweila contributed reporting from Gaza City and Raja Abdulrahim from Jerusalem. Gabby Sobelman contributed additional reporting from Rehovot, Israel.