The attacks came after rockets fired from Palestinian positions killed three Israelis, taking the death toll from Hamas rocket attacks to four since the Israeli offensive began on Saturday.
Wthi the Israeli army amassing infantry and armoured forces along the border in preparation for a possible land invasion, Ban Ki-Moon, the UN secretary-general, added his voice to calls for an end to the violence.
'This must stop'
Speaking at UN headquarters in New York on Monday, he said both sides should end the fighting and said regional powers should do more to help resolve the crisis.
"All this must stop," Ban told a press conference.
"Both Israel and Hamas must halt their acts of violence and take all necessary measures to avoid civilian casualties. A ceasefire must be declared immediately. They must also curb their inflammatory rhetoric."
Israel meanwhile has said it will continue its campaign until it has destroyed Hamas's firepower.
On Monday Ehud Barak, Israel's defence minister, said the country was engaged in a "war to the bitter end" with Hamas which would continue and even intensify until it had achieved its objective.
Speaking to a special session of the Israeli parliament he said Israel was not targeting civilians in Gaza, but was aimed at forcing the territory's Hamas rulers to stop their "hostile actions" against Israeli civilians.
"This operation will expand and deepen as much as needed"
Israeli defence minister
"This operation will expand and deepen as much as needed," Barak said. "We went to war to deal a heavy blow to Hamas, to change the situation in the south."
Earlier Israel declared a "closed military zone" around the Gaza Strip, escalating fears that a ground offensive was imminent.
Israel says the creation of a buffer zone along the border will help protect it from rocket attacks.
Civilians, including journalists, could be banned from an area between 2km and 4km deep under the policy. On previous occasions, such a move has been followed by military operations.
Tanks and troops have been massed in the area since the attacks, referred to by the Israeli military as Operation Cast Lead, began on Saturday.
Al Jazeera's Ayman Mohyeldin, reporting from Gaza City, said that there was little the residents of the strip could do to prepare for any possible ground assault.
"In a city that is so densely-populated, a ground offensive would mean urban warfare, street-to-street fighting ... leaving many Palestinians in the crossfire," he said.
|The Israeli raids have triggered protests around the world [Reuters]
"Unlike other conflict zones where there is the possibility to flee the war zone, Gaza itself has become the war zone. There is nowhere for the population to go, they are in the middle of all these attacks."
Speaking to Al Jazeera on Monday, Tzipi Livni, the Israeli foreign minister, said the Israeli offensive was aimed at Hamas and not the Palestinian people, urging civilians to leave for safer places away from places close to Hamas infrastructure.
"We tried to avoid this. You know that Israel accepted the truce that was initiated by the Egyptians in order to create peace and quiet. We adopted the truce. What we got in return? We got in return daily attacks, we got in return smuggling of weapons to Gaza Strip with long-range [capabilities]," she said.
Support for Israel came from the US, with the White House saying Hamas must halt cross-border rocket fire.
"In order for the violence to stop, Hamas must stop firing rockets into Israel and agree to respect a sustainable and durable ceasefire," Gordon Johndroe, a White House spokesman said.
|Israeli air raids have pounded the heavily-populated Gaza Strip for four days [AFP]
Hospitals in Gaza have been overwhelmed by casualties since Israel started its aerial assault on the territory.
On Monday, Egyptian authorities allowed ambulances carrying several wounded Palestinians to cross into Egypt through the Rafah border crossing for medical treatment.
Egypt also allowed lorries loaded with humanitarian aid to enter its border crossing with the Gaza Strip. Lorries with food and medical supplies had been lining up outside the Egypt-Gaza border since early morning.
Al Jazeera's Sherine Tadros reporting from the Shifa hospital in Gaza City said the situation was chaotic as the territory's health system struggled to cope with the more than 1,400 people injured.
"Hundreds of people are just waiting outside ... the problem is that there simply aren't enough beds to cope with the number of injured," she said.
"Medical sources here are telling us they are running out of everything, from gauzes to saline solutions, and critically now they are running out of almost every type of blood."