GAZA CITY, Gaza Strip — Israel's air force obliterated symbols of Hamas power on the third day of its Gaza assault Monday, striking next to the Hamas premier's home, devastating a security compound and flattening a university building in the deadliest campaign against Palestinians in decades.

Israel's defense minister said his military is fighting a "war to the bitter end" against Hamas but is not fighting the residents of Gaza.

The three-day death toll rose to at least 315 by Monday morning, with some 1,400 wounded. The U.N. said at least 51 of the dead were civilians, and medics said eight children under the age of 17 were killed in two separate strikes overnight. Israel launched its campaign on Saturday in retaliation for rocket fire aimed at southern Israeli towns.

Since then, the number of Israeli troops on the Gaza border has doubled and the Cabinet approved the call-up of 6,500 reserve soldiers.

The strikes have driven Hamas leaders into hiding and appear to have gravely damaged the organization's ability to launch rockets, but barrages continued. Sirens warning of incoming rockets sent Israelis scrambling for cover throughout the day.

One medium-range rocket fired at the Israeli city of Ashkelon killed an Arab construction worker there Monday and wounded several others. He was the second Israeli killed since the beginning of the offensive, and the first person ever to be killed by a rocket in Ashkelon, a city of 120,000.

On Sunday, Hamas missiles struck for the first time near the city of Ashdod, twice as far from Gaza as Ashkelon and only 25 miles (40 kilometers) from Israel's heart in Tel Aviv. Hamas leaders have also threatened to renew suicide attacks inside Israel.

At first light Monday, strong winds blew black smoke from the bombed sites over Gaza City's deserted streets. The air hummed with the buzz of drone aircraft and the roar of jets, punctuated by airstrike explosions. Palestinian health officials said one strike killed four Islamic Jihad militants and a child.

Some Palestinians ventured outside for mourning. In northern Gaza, a father lifted the body of his 4-year-old during a funeral Monday for five children from the same family killed in an Israeli missile strike.

Most of those killed since Saturday were members of Hamas security forces, though the precise numbers remain unclear. A Hamas police spokesman, Ehab Ghussen, said 180 members of the Hamas security forces were among the dead, and the U.N. agency in charge of Palestinian refugees said at least 51 of the dead were civilians. A rise in civilian casualties could intensify international pressure on Israel to abort the offensive.

Israel's intense bombings — more than 300 airstrikes since midday Saturday — reduced dozens of buildings to rubble. The military said naval vessels also bombarded targets from the sea.

One strike destroyed a five-story building in the women's wing at Islamic University, one of the most prominent Hamas symbols in Gaza. Other attacks ravaged a compound controlled by Preventive Security, one of the group's chief security arms, and destroyed a house next to the residence of Ismail Haniyeh, the Hamas prime minister.

Late Sunday, Israeli aircraft attacked a building in the Jebaliya refugee camp next to Gaza City, killing five children and teenagers under age 17 from the same family, Gaza Health Ministry official Dr. Moaiya Hassanain said. In the southern town of Rafah, a toddler and his two teenage brothers were killed in an airstrike aimed at a Hamas commander, Hassanain said. In Gaza City, another attack killed two women.

Some families fled their apartments next to institutions linked to Hamas.

Suad Abu Wadi, 42, kept her six children close on mattresses in her Gaza City living room. Her husband sat with them, chain-smoking. Abu Wadi said he said nothing since seeing their neighbor carrying the body of his child, killed in an airstrike Saturday.

Gaza's nine hospitals were overwhelmed. Hassanain, who keeps a record for the Gaza Health Ministry, said that some of the over 1,400 wounded were now being taken to private clinics and even homes.

Abdel Hafez, a 55-year-old history teacher, waited outside a Gaza City bakery to buy bread. He said he was not a Hamas supporter but believed the strikes would only increase support for the group. "Each strike, each drop of blood are giving Hamas more fuel to continue," he said.

Ehud Barak, the Israeli defense minister, told parliament Israel was not fighting the residents of Gaza. "But we have a war to the bitter end against Hamas and its branches," he said. Barak said the goal is to deal Hamas a "severe blow" and that the operation would be "widened and deepened as needed."

In Israel, 17 people have been killed in attacks from Gaza since the beginning of the year, including nine civilians — six of them killed by rockets — and eight soldiers, according to Israel's Foreign Ministry.

Israeli security officials have warned that the militants' range now includes Beersheba, a major city 30 miles (50 kilometers) from Gaza. Resident Mazal Ivgi, 62, said she had prepared a bomb shelter. "In the meantime we don't really believe it's going to happen, but when the first boom comes people will be worried," she said.

Israel began Saturday's assault by targeting Hamas security installations, and has broadened the attacks since then. On Sunday planes struck dozens of smuggling tunnels under the Gaza-Egypt border, cutting off a key lifeline that had supplied Hamas with weapons and Gaza with commercial goods.

In Jerusalem, Israel's Cabinet approved a call-up of 6,500 reserve soldiers Sunday in apparent preparation for a ground offensive. The final decision to call up reserves has yet to be made by the defense minister, and the Cabinet decision could be a pressure tactic. Military experts said Israel would need at least 10,000 soldiers for a full-scale invasion.

Israel has doubled the number of troops around Gaza and also deployed an artillery battery. Several hundred reservists have already been summoned to join their units, but no full combat formations have been mobilized so far.

The assault has sparked diplomatic fallout. Syria decided to suspend indirect peace talks with Israel, and the U.N. Security Council called on both sides to halt the fighting and asked Israel to allow humanitarian supplies into Gaza. Israel opened one of Gaza's border crossings Monday, and about 40 trucks had entered with food and medical supplies by midday, military spokesman Peter Lerner said.

Egypt also opened its borders to Gaza and allowed trucks loaded with humanitarian aid to enter the Rafah terminal Monday. It was also taking in wounded Palestinians from Gaza, with more than a dozen Egyptian ambulances waiting at the crossing.

The Palestinian president, Mahmoud Abbas, who heads a moderate government in the West Bank and is holding peace talks with Israel, issued his strongest condemnation yet of the operation, calling it a "sweeping Israeli aggression against Gaza" and saying he would consult with his bitter rivals in Hamas in an effort to end it.

Israel is trying to avoid civilian casualties, Foreign Minister Tzipi Livni told reporters Monday, while "Hamas is looking for children to kill."

"Hamas is targeting deliberately kindergartens and schools and citizens and civilians because this is according to their values. Our values are completely different. We are trying to target Hamas, which hides among civilians," Livni said.

The carnage inflamed Arab and Muslim public opinion, setting off street protests in Arab communities in Israel and the West Bank, across the Arab world, and in some European cities.

In Iraq, about 1,000 backers of anti-American Shiite cleric Muqtada al-Sadr staged a protest Monday in Baghdad demanding Israel immediately stop its air assault. The political party of Iraqi Prime Minister Nouri al-Maliki condemned the attacks and called on Islamic countries to cut relations with Israel.

On Monday, a Palestinian stabbed and wounded four Israelis in a West Bank settlement before he was shot and wounded. It was not immediately clear if the attack was directly connected to the events in Gaza.