GAZA CITY, Gaza Strip — Israel's air force obliterated symbols of Hamas power on the third day of its Gaza assault today, striking next to the Hamas premier's home, devastating a security compound and flattening a building at a university linked to the Islamic group.

Israel's defense minister said his military is fighting a "war to the bitter end" against Hamas.

The three-day death toll rose to at least 315 by this 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, the deadliest against Palestinians in decades, on Saturday in retaliation for rocket fire aimed at civilians in 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 today 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 today, 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.

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.