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Israeli officials said that many of the rocket arsenals are deliberately located adjacent to schools and mosques, but that all efforts would be made to avoid civilian casualties.

Hamas officials said 20 sites were hit in the first two hours of the airstrikes.

Leaders of Hamas and other militant organizations said Israel had “declared war” on Gaza and that retaliation would be fierce. The first significant response was a heavy barrage of rockets fired at Beersheba and other cities.

They fired rockets for the first time at Dimona, where Israel’s nuclear reactor is located, but the missiles fell far short of their target, Israeli officials said. At least 15 rockets were intercepted by Israel’s Iron Dome anti-missile defense system.

Egyptian President Mohammed Morsi recalled the Egyptian ambassador to Israel and summoned Israel’s ambassador in Cairo to complain about the attacks. Mr. Morsi also asked the Arab League to convene an emergency meeting of Arab foreign ministers.

At Egypt’s urging, the U.N. Security Council held an emergency closed-door meeting Wednesday night, also involving the Israeli and Palestinian representatives to the body. Arab ambassadors want the council “to condemn that barbaric attack,” but the U.S. likely would veto such an declaration.

Mr. Obama also spoke to Mr. Morsi about “Egypt’s central role in preserving regional security,” the White House said. The two men “agreed on the importance of working to de-escalate the situation as quickly as possible and agreed to stay in close touch in the days ahead.”

Hamas officials said targets throughout Israel will be hit by rockets and suicide bombers. The Israeli army announced a limited mobilization of reservists to help shore up the home front, and police erected roadblocks at the entrances to cities.

In recent days, Israeli communities have been subjected to heavy rocketing, with more than 100 rockets and mortar shells on Sunday alone. Mr. Netanyahu summoned foreign ambassadors and said that no nation could tolerate the continuous assault on its cities.

On Wednesday, Mr. Netanyahu, Mr. Barak and Chief of Staff Gen. Benny Gantz traveled to the Golan Heights, where shells from Syria have landed in recent days.

El-Jaabari was considered the most authoritative and respected figure in Hamas, even more than the elected leadership. He is credited with turning a ragtag collection of terrorists into an organized, quasi-military force.

El-Jaabari was behind the kidnapping of Israeli soldier Gilad Shalit, whom Hamas kidnapped in 2006 and released in a prisoner exchange with Israel last year.

This article is based in part on wire service reports.