JERUSALEM — Israel braced Wednesday for retaliation from Palestinian militants, including possible suicide bombings within the Jewish state, after killing Hamas' military chief in an airstrike in Gaza City.
The assassination of Ahmed el-Jaabari, a founding member of the Islamist terrorist group who topped Israel's target list for years, marked Israel's most significant action against Palestinian militants since its incursion into the Gaza Strip four years ago.
Israel also launched intensive air attacks on Hamas' military infrastructure throughout the Gaza Strip, particularly underground storage areas for long-range Fajr rockets. With a range of 46 miles, the rockets are capable of hitting the Tel Aviv metropolitan area.
"Today, we sent a clear message to Hamas and other terrorist organizations," Israeli Prime Minister Benjamin Netanyahu said during a televised address. "If it becomes necessary, we are prepared to expand the operation."
In Gaza, Hamas' armed wing, the Ezzedine al Qassam Brigades, issued a furious statement warning that Israel has "opened the gates of hell on itself."
The United States supported the Israeli action as a right of self-defense, though President Obama asked Mr. Netanyahu to minimize civilian casualties.
"The president urged Prime Minister Netanyahu to make every effort to avoid civilian casualties," the White House said Wednesday night. "The two agreed that Hamas needs to stop its attacks on Israel to allow the situation to de-escalate."
Defense Minister Ehud Barak said the operation was aimed at strengthening Israel's deterrence, damaging militant groups' rocket-firing capabilities and stamping out attacks on southern Israel that have increased dramatically over the past month.
"Israel doesn't want a war, but the Hamas provocation of recent weeks forced us to act sharply and decisively," Mr. Barak said. "We are at the beginning, not end, of this action. It won't be a quick fix."
The attacks caused "significant damage" to the Fajr arsenal, but Israeli officials do not know whether all the missiles were destroyed, said Gen. Yoav Mordechai, a spokesman for the Israel Defense Forces.
Terrorist organizations in Gaza are thought to have 10,000 rockets capable of hitting targets up to 25 miles away, including the cities of Beersheba, Ashkelon and Ashdod, all of which have been hit frequently.
Schools within range of Gaza rockets will be closed as long as the Israeli military operation continues, officials said. Residents were advised to stay close to shelters.
Israeli officials stressed that the operation has no time limit. They said it is aimed at restoring Israel's deterrence and eliminating the strategic threat from rockets of increasing range smuggled into Gaza, especially from Iran.
"Gaza has become a forward base for Iran," said Gen. Mordechai. "The days ahead of us will not be simple."
Israel is prepared to launch a ground incursion into Gaza if necessary, but it is now confining operations to air attacks that are "as surgical as possible," he said.
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.