JERUSALEM -- Israeli forces expanded their offensive deeper into the Gaza Strip on Sunday, heavily shelling a densely populated neighborhood of Gaza City and killing at least 50 people, local medical officials said.
Palestinians called the shelling of Shuja'iya, and adjacent areas in the eastern part of Gaza City, a "massacre." Television images from the neighborhood showed bodies strewn in the streets and a stream of panicked residents fleeing areas under attack.
The Israeli army, which in recent days warned local residents to leave, said troops advancing toward the neighborhood had called in artillery support after meeting stiff resistance from Palestinian gunmen fighting from houses.
The episode, in which more than 200 people were reportedly injured, appeared to be the deadliest since Israel launched its offensive in Gaza on July 8 against the militant Islamist group Hamas.
After hours of artillery shelling, the Israeli army agreed to a request by the Red Cross for a two-hour pause in the bombardment to allow evacuation of the wounded and recovery of bodies.
However, an Israeli army spokesman said that Hamas had violated the ceasefire and that Israeli forces were "responding accordingly." He said the military had called on residents to evacuate the area toward the center of Gaza City.
Many residents fleeing Shuja'iya sought shelter in Al-Shifa Hospital in the city center, Palestinians reported. Images from the hospital showed distraught men carrying bodies of two dead children.
Plumes of smoke rose from the bombarded area, and reports from the scene said that ambulance crews were unable to get in to look for dead and wounded feared trapped under the rubble of collapsed houses.
Israeli Prime Minister Benjamin Netanyahu has said that the aim of the Israeli military campaign is to bring an extended halt to rocket fire at Israel by Gaza militants and to strike a heavy blow against Hamas.
But the offensive has triggered heavy rocket-firing from Gaza at Israel, with rockets reaching the area of Tel Aviv and the northern port city of Haifa, 100 miles away, sending Israelis across the country to safe-rooms and shelters.
Many of the incoming rockets have landed in open areas, while others have been intercepted by Israel's Iron Dome missile defense system.
More than 350 Palestinians have been killed in the Israeli bombardments and more than 1,500 wounded, according to local medical officials. The United Nations said about 70 percent of the dead were civilians, at least 77 of them children. The Israeli army says it has killed about 70 militants in ground operations.
Two Israeli civilians have been killed by rocket attacks from Gaza, and at least five soldiers have died in fighting on the ground.
After 10 days of bombardment from land, sea and air, Israeli troops and tanks crossed into Gaza on Thursday for what Netanyahu said was a mission to uncover and destroy tunnels dug by Hamas, some extending under the border into Israel.
After initially occupying sparsely populated areas up to two miles from the border, the army expanded its operations toward built-up neighborhoods of Gaza City on Sunday, raising the prospect of increasing casualties on both sides caused by close-quarters urban warfare. . Lt. Col. Peter Lerner, an Israeli army spokesman, said that Israeli forces had been moving toward Shuja'iya, which he called a "Hamas stronghold" from which about 140 rockets had been fired at Israel since the start of the Israeli offensive.
The forces were "on the way to deal with tunnels they have in Shuja'iya and to deal with rocket launching from there," Lerner said. "We met extensive resistance from Hamas terrorists shooting at forces from houses in the neighborhood, firing rocket-propelled grenades from several directions. Artillery was fired in support, to address that challenge."
Lerner said there were casualties among Israeli troops, but he declined to provide details. Military announcements of soldiers killed in action are routinely delayed to allow for notification of their families.
Four Israeli soldiers were killed on Saturday. Two died when their jeep was hit by an anti-tank rocket fired by Hamas gunmen who had tunneled across the border into Israel. A bulldozer driver from a combat engineering unit was killed when his vehicle was hit by an anti-tank rocket, and another soldier died in a firefight with militants, the army said.
Tens of thousands of Palestinians, some urged by Israeli army warnings, have fled their homes to escape the fighting in Gaza. The United Nations Relief and Works Agency, which assists Palestinian refugees, said that more than 63,000 people had sought refuge in 55 shelters provided by the agency, many of them schools.
"The number has tripled in the last three days, reflecting the intensity of the conflict and the inordinate threats the fighting is posing to civilians," said UNRWA spokesman Chris Gunness.
Efforts to negotiate a ceasefire continued on Sunday, with Palestinian Authority President Mahmoud Abbas due to meet Khaled Mashal, the political leader of Hamas, in Qatar.
Hamas has rejected a ceasefire proposal by Egypt, saying it was not consulted about its terms. The plan called for a halt in fighting followed by truce negotiations, but Hamas said it wanted it demands addressed first, primarily the easing of border closures imposed on Gaza by Egypt and Israel.