Israel said the airstrikes, launched in response to days of rocket fire out of Hamas-ruled Gaza, were the beginning of a broader operation against the Islamic militants codenamed "Pillar of Defense." Israeli defense officials said a ground operation was a strong possibility in the coming days though they stressed no decisions had been made and much would depend on Hamas' reaction. There were no immediate signs of extraordinary troop deployments along the border.
The attack came at a time when Israel seems to be under fire from all directions. Relations have been deteriorating with Egypt's new Islamist government, Egypt's lawless Sinai desert has become a staging ground for militant attacks on Israel, and the Syrian civil war has begun to spill over Israel's northern border. Earlier this week, Israel fired back at Syria -- for the first time in nearly 40 years -- after stray mortar fire landed in the Israeli-controlled Golan Heights.
With at least 10 Palestinians dead, including two young children, Wednesday's offensive was certain to set off a new round of heavy fighting with Gaza militants, who have built up a formidable arsenal of rockets and missiles.
It also threatened to upset Israel's relations with neighboring Egypt and shake up the campaign for Israeli elections in January. In a preliminary response, Egypt recalled its ambassador to Israel in protest.
In a nationwide address, Prime Minister Benjamin Netanyahu said Israel could no longer stand repeated attacks on its southern towns. Days of rocket fire have heavily disrupted life for some 1 million people in the region, canceling school and forcing residents to remain indoors.
"If there is a need, the military is prepared to expand the operation. We will continue to do everything to protect our citizens," Netanyahu declared.
The Israeli military said it was ready, if necessary, to send ground troops into Gaza. The defense officials who said a ground operation was likely in the coming days spoke on condition of anonymity because they were discussing sensitive military plans.
"We are at the beginning of the event, and not the end," Defense Minister Ehud Barak said, in a joint appearance with the prime minister. "In the long run I believe the operation will help strengthen the power of deterrence and to return quiet to the south." In a sign that the operation was expected to broaden, the military was cleared to call up reserve units.
Residents in both Israel and Gaza braced for prolonged violence. Gazans rushed to stock up on food and fuel. After nightfall, streets were empty as the sounds of Israeli warplanes and explosions of airstrikes could be heard in the distance.
Israel declared a state of emergency in its south and canceled school across the area for Thursday. Calling it a "special situation," Barak sought permission to call up special reserve units for the operations. Israeli police stepped up patrols around the country, fearing that Hamas could retaliate with bombing attacks far from the reaches of Gaza.
Hamas has in the past staged dozens of suicide bombings against Israelis and while its capabilities to do so today have been curtailed by Israeli and Palestinian crackdowns, it still has a network in the West Bank.
More than 65 rockets landed in southern Israel late Wednesday. One projectile struck a shopping mall in the southern city of Beersheba, causing heavy damage but no casualties, police said.
The Israeli military said 25 rockets were intercepted by the "Iron Dome" rocket-defense system. Israeli media said the rockets had been headed toward Beersheba. Israeli aircraft continued to pound Gaza into the night with some 50 airstrikes, with no reports of casualties.
The deadly attack on Hamas mastermind Ahmed Jabari marked the resumption of Israel's policy of "targeted killings," or assassinations of senior Hamas men. Israel has refrained from such attacks, which have drawn international condemnations, since a fierce three-week offensive in Gaza that ended in January 2009.
The earlier Gaza offensive killed more than 1,400 Palestinians, including hundreds of civilians. Israel has blamed Hamas for the heavy civilian casualties, accusing the group of using schools and residential neighborhoods as cover. Nonetheless, Israel was harshly criticized internationally for the heavy civilian death toll.
Jabari was the most senior Hamas official to be killed since that war. He had long topped Israel's most-wanted list, blamed for masterminding a string of deadly attacks that including a bold, cross-border kidnapping of an Israeli soldier in 2006. He also was believed to be a key player in Hamas' takeover of Gaza in 2007 from a rival Palestinian faction, the Western-backed Fatah movement.
"I would call him the No. 1 terrorist in the Gaza Strip, whose hands are stained with blood," said Brig. Gen. Yoav Mordechai, Israel's chief military spokesman.
Israel and Hamas have largely observed an informal truce for the past four years.
But in recent weeks, the calm has unraveled in a bout of rocket attacks out of Gaza and retaliatory Israeli airstrikes. From Israel's perspective, Hamas escalated tit-for-tat fighting in recent days with a pair of attacks: an explosion in a tunnel along the Israeli border and a missile attack on an Israeli military jeep that seriously wounded four soldiers.
Israeli defense officials warned earlier this week that they were considering resuming the assassination policy.
Even so, the Jabari killing, carried out in broad daylight, was shocking. Hamas officials had brushed off the Israeli threats, illustrated by Jabari's decision to drive in public. Hamas leaders typically go into hiding at times of rising tensions. Over the past two days, the fighting had shown signs of petering out as Egyptian mediators tried to broker a truce.
The Israeli military released a black-and-white video of the airstrike, showing a sedan moving slowly along a road before going up in flames in an explosion so powerful that a large chunk of the vehicle flew high into the air.
Crowds of people and security personnel rushed to the scene of the strike, trying to put out the fire that had engulfed the car and left it a charred shell. Plumes of black smoke wafted into Gaza City's skies following other airstrikes. Ambulance sirens blared as people ran in panic in the streets and militants fired angrily into the air.
The Israeli military also released footage of its strikes against weapons depots and rocket-launching grounds. Barak said these airstrikes hit "terror infrastructure" and launchers used to fire Iranian-made Fajr rockets. The rockets, capable of reaching Tel Aviv, are among Hamas' most powerful weapons.
The military said it hit dozens of medium-range rocket launch sites, many of which are located in residential buildings, military officials said. They spoke on condition of anonymity because they were not authorized to discuss the operation. The military also said the navy was striking Hamas targets located by the shore. Hamas denied that any of its weapons stores were hit.
Hamas announced a state of emergency in Gaza. It evacuated all its security buildings and deployed its troops away from their locations.
Outside the hospital where Jabari's body was taken, thousands of Gazans chanted "Retaliation!" and "We want you to hit Tel Aviv tonight!"
"I was sitting on my bed with my grandson when suddenly the wall collapsed on both of our heads," said Mahmoud Bana, a 62-year-old man who was slightly wounded along with his 11-year-old grandson. "We don't know what happened but we know it is going to be a few hard days ahead."
In a statement, Hamas' prime minister, Ismail Haniyeh, eulogized Jabari and vowed revenge.
"We mourn our late leader who walked the path of jihad while he knew the end, either victory or martyrdom," Haniyeh said. "There is no fear among our people and our resistance, and we will face this vicious attack."
The airstrike bore many similarities to the start of Israel's previous offensive in December 2008. That operation also began with an air raid on Hamas buildings, and also took place in between American presidential elections and Israeli parliamentary elections.
Hamas accused Netanyahu of launching Wednesday's operation to win votes in the Jan. 22 parliamentary election. But major Israeli parties, including the dovish opposition, all lined up behind Netanyahu.
Still, the region has changed greatly over the past four years. Most critically for Israel, Egypt is now governed by Hamas' ideological counterpart, the Muslim Brotherhood.
Israel and Egypt signed a peace accord in 1979. Relations, never warm, have deteriorated since longtime Egyptian President Hosni Mubarak was ousted in a popular uprising last year. The assassination threatened to further damage those fraying ties.
On its official Facebook page, the Freedom and Justice Party, the Muslim Brotherhood's political arm, called Jabari's assassination a "crime that requires a quick Arab and international response to stem these massacres against the besieged Palestinian people in the Gaza Strip."
It accused Israel of trying to "drag the region toward instability."
In Washington, the United States lined up behind Israel. "We support Israel's right to defend itself, and we encourage Israel to continue to take every effort to avoid civilian casualties," said State Department spokesman Mark Toner. He denounced Hamas rocket attacks.
Netanyahu spoke to Obama and thanked him for the support, said a statement from his office.
U.N. Secretary-General Ban Ki-moon called for calm and urged both sides to respect international humanitarian law.
Israel's use of targeted killings is one of the most contentious policies used against militants.
Advocates say targeted killings are an effective deterrent without the complications associated with a ground operation, chiefly civilian and Israeli troop casualties. Proponents argue they also prevent future attacks by removing their masterminds.
Critics say the killings invite retaliation by militants and encourage them to try to assassinate Israeli leaders. They complain that the strikes amount to extrajudicial killings.
During a wave of suicide bombings against Israel a decade ago, the country employed the tactic to eliminate the upper echelon of Hamas leadership.
During that period, Israeli aircraft assassinated the previous commander of Hamas' military wing, Salah Shehadeh, the movement's founder and spiritual leader, Sheik Ahmed Yassin, his successor, Abdel Aziz Rantisi, and dozens of other Hamas military commanders.
The practice set off a wave of criticism from rights groups and foreign governments, particularly the strike that killed Shehadeh -- a one-ton bomb that killed 14 other people, most of them children.
Pro-Palestinian groups have attempted, unsuccessfully, to arrest Israeli officials involved in the Shehadeh killing on war crimes charges. While charges have never been filed, fears of arrest have forced a number of Israeli officials to cancel travel to Europe over the years.