Gaza City (CNN) -- Hundreds of Palestinians fled in panic into Gaza City on Sunday as Israeli troops focused their firepower on the nearby town of Shaja'ia. The shelling and bombing killed at least 60 people and wounded 300, according to the Gaza Health Ministry.
In previous days, Israel warned residents to flee, through calls, text messages and dropping fliers that said "it is the intention of the IDF to carry out aerial strikes against terror sites and operatives" in the area. The fliers told people to head to Gaza City by Wednesday morning and not to return until further notice. The IDF posted an English translation of the fliers Sunday on Twitter.
Some residents said they had received the warnings but felt that even if they fled, they could face the same dangers in other parts of Gaza.
But the IDF said Hamas "ordered them to stay" and "put them in the line of fire."
More than 410 people in Gaza have died in the fighting between Israel and Hamas, Palestinian officials say.
Militants have launched about 300 rockets into Israel since the beginning of Israel's ground operation, the IDF said. More than 200 hit Israel, and 64 were taken out by the Iron Dome defense system.
On Saturday, a rocket killed an Israeli in Negev and wounded four others, including an infant, Israeli authorities said.
Two Israeli civilians have been killed by Hamas rocket or mortar fire in the past week's warfare. Five soldiers were killed and two were severely wounded.
The IDF is adding more troops to the incursion. It called up tens of thousands of reservists at the start of Operation Protective Edge to prepare for the ground operations.
Israel said it has struck "2,300 terror targets" in Gaza and found 13 tunnels the militants use for smuggling weapons.
Since beginning ground operations Thursday, Israel said, it has killed at least 70 terrorists and captured others.
U.S. may dispatch Kerry
U.S. Secretary of State John Kerry told CNN Sunday he may travel to the region.
"I believe the President is asking me to go over there in very short order to work on the issue of a cease-fire," he told CNN's "State of the Union."
"Israel is under siege by a terrorist organization that has seen fit to dig tunnels and come through those tunnels with handcuffs and tranquilizer drugs, prepared to try to capture Israeli citizens and take them back to hold them hostage. No country could sit by and not take steps to try to deal with people who are sending thousands of rockets your way," Kerry said.
During one phone call with President Obama, Israeli Prime Minister Benjamin Netanyahu "had to interrupt the conversation with the President of the United States to go to a shelter. People can't live that way," Kerry said.
The United States supports Egypt's initiative for a cease-fire and "will work for a fair cease-fire," he said. The United States has "shown our willingness to try to deal with the underlying issues," but Hamas "must step up and show a level of reasonableness," he said.
"No country, no human being, is comfortable with children being killed, with people being killed, but we're not comfortable with Israeli soldiers being killed either or with people being rocketed in Israel."
"Hamas uses civilians as shields," he said. "They fire from a home and draw the fire into the home."
Israel opening field hospital
Israel announced Sunday it would open a field hospital at the Erez Crossing to treat injured Palestinians. On Saturday, the defense forces delivered truckloads of medical supplies to Gaza.
Overnight, Hamas fired rockets from Shaja'ia toward Israel. The militant group also turned down an invitation by Egypt to talk about a cease-fire initiative that Cairo had proposed.
Fawzi Barhoum, a spokesman for Hamas, said on Aqsa TV on Saturday that there would be no truce or surrender while Israel is attacking.
Israel agreed to a two-hour cease-fire Sunday, at the request of the Red Cross, to allow Palestinian emergency medical workers to tend to the wounded and dead in Shaja'ia, the IDF said. But the fighting continued.
The IDF blamed Hamas, saying the militant group fired at Israeli ground forces.
Hamas blamed Israel, saying its forces shelled Shaja'ia, striking ambulances that tried to enter the area.
Netanyahu: Demilitarize Gaza
Netanyahu called on the international community to "undertake a program to demilitarize Gaza" in the future.
The situation is "unacceptable" because of Hamas and Islamic Jihad, Netanyahu told CNN's Wolf Blitzer in an interview Sunday
"These people are the worst terrorists -- genocidal terrorists. They call for the destruction of Israel and they call for the killing of every Jew, wherever they can find them."
Hamas fighters in Gaza "don't care" about the dying people around them, Netanyahu said.
Israel has enabled the shipment of concrete into Gaza for buildings, hospitals, and schools, but the militants use hundreds of tons of it for each tunnel, Netanyahu said.
Hamas spokesman Sami Abu Zuhri, speaking to Al-Jazeera, said Israel committed "a crime against humanity," and that most of those killed in Shaja'ia were women and children. "Our people will not sit idle in front of this brutal aggression."
He called on the Palestinian Authority, in the West Bank, to "stop its security coordination with the occupation" and to "stop suppressing the demonstrations in the West Bank." He also said "the Arab world should not sit idle."
In stricken town, bodies in streets
In Shaja'ia, bodies lay in streets beside gashes blasted into apartment buildings, said people who had escaped the violence.
A towering column of black smoke rose over the town, and video showed a helicopter gunship overhead. Ambulances rushed up to take the wounded to hospitals, where moments later, they were rolled into triage.
Many of the wounded were hit inside their homes, the Gaza Health Ministry said. Incoming rounds struck some ambulances and hindered paramedics from picking up some of the injured.
One paramedic died in the shelling, Gaza Health Ministry spokesman Dr. Ashraf al-Qedra said on Hamas-run Aqsa TV. He said injured people were waiting on hospital floors and in hallways for treatment.
On the Israeli side of the border, the sound of outgoing artillery fire thundered every few seconds, as tanks and other military vehicles gathered for a potentially larger incursion.
The IDF is adding more troops to the ground incursion it began on Thursday, after more than a week of airstrikes.
The Israeli government has repeatedly said that, unlike Palestinian militants, the IDF does not target civilians and works to avoid innocent casualties.
But in Gaza, one of the most densely populated areas on Earth, more than 70% of those killed in the hail of artillery and airstrikes have been civilians, according to the United Nations.
A fifth were children. More than 40% of Gaza's population is 14 years old or younger.
About 70,000 Palestinians have taken refuge in U.N. schools, Robert Turner, the director of U.N. efforts in Gaza, said Sunday. The United Nations has been investigating a cache of rockets used by militants found in a U.N. school.
Leaders from the United States and Europe last week expressed public support for Israel's right to defend itself against missiles from Gaza.