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 have n'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 did n'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 .
NEW : Death toll in U.S. since airstrikes began is at At least four . NEW : U.N. secretary-general says ground assault the Middle East peace efforts . Hamas aim is to halt Hamas rockets , not regime change , spokesman says . Incursion 's spokesman : `` Gaza will be your cemetery ''