JERUSALEM (CNN) -- Blasts of heavy machine-gun fire and explosions from airstrikes filled the air in Gaza early Sunday morning, hours after Israel rolled thousands of troops into the Palestinian territory.

Israeli soldiers on the Israeli-Gaza border wait to enter Gaza on Saturday night.

At least four Palestinians have been killed as a direct result of the ground assault, Palestinian medical sources said, bringing the death toll in Gaza to 460 since the conflict erupted December 27.

The ground assault followed a weeklong campaign of airstrikes in what Israel says is an offensive to halt rocket attacks from Hamas militants in Gaza.

"We want to create a situation where the civilian population in southern Israel is no longer on the receiving end of those deadly Hamas rockets," Israeli government spokesman Mark Regev told CNN.

"We haven't articulated regime change as the goal of this operation. Our goal is to protect our people," Regev said.

Israeli officials say four people have been killed and 59 wounded in Hamas rocket attacks in the past week.

As night fell on the eighth day of the bloody conflict, both sides remained defiant, vowing to stand their ground and accusing the other of breaking a cease-fire that had been in place until last month. Watch the conflict unfold on both sides »

Not long after Israel's ground incursion began, Hamas vowed to "fight until the last breath" and warned Israel that "Gaza will be your cemetery."

As the ground attack began, Hamas rockets continued to fly into southern Israel, with CNN reporters at the Israel-Gaza border seeing and hearing several of them.

"We will not abandon the battlefield, and we will stay on the thorny course, and we will fight until the last breath," Hamas chief spokesman Ismail Radwan said in a statement on Palestinian network Al-Aqsa. Watch statement from Hamas »

The Israeli ground attack deepened a humanitarian crisis in Gaza that has been decried by international observers, including the United Nations.

On Saturday, U.N. Secretary-General Ban Ki-moon spoke with Israeli Prime Minister Ehud Olmert, a spokesman said. View reactions from world leaders »

The spokesman said Ban "is convinced and alarmed that this escalation will inevitably increase the already heavy suffering of the affected civilian populations," according to a statement.

Electricity, food and medical shortages already were a problem in the densely populated territory, concerns ramped up by the airstrikes.

Taghreed El-Khodary, a New York Times writer in Gaza City, said that there was no electricity there Saturday and that she said she didn't know of any shelters set up by Hamas for residents. Watch Times writer describe the humanitarian crisis in Gaza »

Regev said Gaza's civilian population was not Israel's enemy.

"In many ways, they are victims like us. Both the civilian population of southern Israel and the civilian population of the Gaza Strip have been victims of this terrible, extremist Hamas regime," Regev said. Watch CNN's Christiane Amanpour interview Regev »

An Israeli airstrike hit a mosque in the northern Gaza village of Beit Lahiya on Saturday evening, killing 13 people and wounding 60, according to Palestinian medical sources. Watch the chaos after the mosque attack »

An Israeli attack earlier Saturday killed Azkariah al-Jamal, commander of Gaza City's rocket-launching squads, Palestinian and Israel Defense Forces sources said.

Another airstrike killed two Hamas militants in Khan Yunis, in southern Gaza, Palestinian officials said.

Israeli missiles hit 25 Hamas "outposts, training camps and rocket launching sites" Saturday morning, an IDF statement said. Watch explosions in Gaza »

The houses of two alleged Hamas terrorist operatives were also hit, the IDF said. One was the home of Azadin Hadad, described as the head of the Hamas military group in eastern Gaza City.

The other, in Beit Lahiya, was used to store rocket-launching equipment and was the home of Ismail Renam, who "has a central role in the launching of Grad-type rockets against Israel," the IDF said.

Meanwhile, Hamas militants launched 20 rockets from Gaza into Israel by late afternoon Saturday, IDF spokeswoman Maj. Avital Leibovich said.

Rockets set a house on fire in Ashkelon, damaged a kibbutz dining hall and slightly wounded two people in an eight-story building in Ashdod, according to Israeli ambulance services.

"We still see that Hamas is targeting Israeli civilians. Namely, we still did not reach the goal of crippling Hamas' launching capabilities," Leibovich said.

CNN's Barbara Starr said top U.S. military leaders were aware in advance of Israel's plans to enter Gaza.

The incursion began just hours after the European Union announced that a delegation is heading to the Middle East to meet with regional leaders and broker a cease-fire.

A Middle East "Quartet" consisting of the European Union, the United States, the United Nations and Russia is also trying to broker a truce.

Hamas, an Islamic fundamentalist group that came into being in 1987, won by a landslide in parliamentary elections in Gaza in 2006. Months later, Hamas militants began attacking Israeli military posts and fighting with members of Fatah, the rival political party that controls other Palestinian territories.

Palestinian Authority President Mahmoud Abbas declared a state of emergency and dissolved the government in Gaza, although Hamas has had de facto control of the territory since then.

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