Palestinians in Gaza say Israeli bombardment is constant and there is no place to escape. Heavy fighting is raging in the Gaza Strip between Israeli forces and Palestinian fighters after Israel launched its ground offensive on the territory. At least 30 Palestinians, mostly civilians, have been killed since fighting began late on Saturday, according to Palestinian medical sources. An Israeli soldier was killed on Sunday in the fighting, the Israeli army said. Thousands of Israeli troops entered the Gaza Strip overnight with tanks and helicopters, accompanied by naval support and air strikes. The ground offensive followed eight days of intense Israeli bombardment of the Gaza Strip, purportedly aimed at ending cross-border rockets being fired by Hamas fighters from the territory into southern Israel. The Israeli assault, codenamed "Operation Cast Lead", has killed more than 507 Palestinians and wounded more than 2,400 others. Four Israelis have been killed by the Hamas rocket strikes in the same time. An undaunted Hamas, however, has vowed to fight back and defeat the Israeli forces. A spokesman for Ezzedine al-Qassam Brigades, the armed wing of the Hamas, told Al Jazeera on Sunday that Israeli troops faced death or capture. "The battle has just started and the enemy should endure the consequences and results. They should be ready for the bad news coming from the Gaza Strip," Abu Obeida, a spokesman, said. Hamas said it had captured two Israeli soldiers but the Israeli army denied that. While the UN secretary-general called for an immediate end to the operations, the Security Council failed to agree on a resolution calling for a ceasefire after an emergency meeting. Shimon Peres, the Israeli president, has meanwhile rejected the possibility of a ceasefire but said Israel does not intend to occupy Gaza. "We don't intend neither to occupy Gaza nor to crush Hamas, but to crush terror. And Hamas needs a real and serious lesson. They are now getting it," Peres said in an interview. Al Jazeera's Sherine Tadros, reporting from the Shifa hospital in Gaza City, said doctors were struggling to cope amid low supplies and the rising number of wounded. She said the scene was chaotic, with doctors treating the injured on the floor. Fears of a humanitarian crisis have also grown in recent days, as the strip, home to 1.5 million people, is already suffering shortages of fuel, food and medical supplies due to a two-year economic blockade imposed by Israel. The International Committee for the Red Cross said on Sunday its medical emergency team had been prevented for a third day from entering the territory. Egypt has also completely closed the Rafah crossing, cutting off aid supplies to the territory. The UN has warned that there were "critical gaps" in aid reaching Gaza, despite claims from Tzipi Livni, the Israeli foreign minister, that there was no crisis and that aid was getting through. However, Christopher Gunness, the UN Relief and Works Agency (Unrwa) spokesman said the idea that there is no humanitarian crisis is absurd. "The organization for which I work - Unrwa - has approximately 9-10,000 workers on the ground. They are speaking with the ordinary civilians in Gaza... People are suffering. A quarter of all those being killed now are civilians. So when I hear people say we're doing our best to avoid civilian casualties that rings very hollow indeed." Elsewhere in the strip, heavy artillery, tracer fire and rockets could be heard while reports said Israeli troops had reached the northern towns of Beit Lahiya and Beit Hanoun. Soldiers and fighters were also locked in gun battles east of the Hamas stronghold of Zeitoun.