GAZA CITY, Gaza Strip (AP) -- Israeli troops pushed deeper into Gaza on Friday in a ground offensive that officials said could last up to two weeks as the prime minister ordered the military to prepare for a "significantly" wider campaign.
The assault raised risks of a bloodier conflict amid escalating Palestinian civilian casualties and the first Israeli military death -- and brought questions of how far Israel will go to cripple Gaza's Hamas rulers.
Officially, the goal remains to destroy a network of tunnels militants use to infiltrate Israel and attack civilians. In its first day on the ground in Gaza, the military said it took up positions beyond the border, encountered little resistance from Hamas fighters and made steady progress in destroying the tunnels. Military officials said the quick work means that within a day or two, Israeli leaders may already have to decide whether to expand the operation.
With calls from Israeli hard-liners to completely crush Hamas, it remains unclear how far Israel will go in an operation that has already seen 299 Palestinians killed in 11 days of intense Israeli bombardment of the densely populated coastal strip, a fifth of them children.
Israeli Prime Minister Benjamin Netanyahu said he had ordered the military to prepare for a "significant expansion" of the ground offensive.
"It is not possible to deal with tunnels only from the air. It needs to be done also from the ground," he told a special Cabinet meeting in Tel Aviv. "We chose to begin this operation after the other options were exhausted and with the understanding that without the operation, the price we will pay can be very high."
Frustrated by Hamas' refusal to accept an Egyptian-brokered truce agreement and the failure of a 10-day campaign of more than 2,000 airstrikes to halt relentless rocket fire on Israeli cities, Israel launched a ground offensive it had previously been reticent to undertake to further weaken Hamas militarily.
"It won't end that quickly," said Yitzhak Aharonovitch, Israel's minister of public security. "Anything can happen. If we need to keep going, we will keep going. We won't stop. We need quiet for the citizens of the south and the citizens of Israel."
In a fresh effort to broker a truce, U.N. chief Ban Ki-moon was to leave Saturday for the Middle East to help mediate the Gaza conflict, U.N. officials said. A cease-fire is "indispensable" for urgently needed humanitarian efforts to succeed, the under-secretary-general for political affairs Jeffrey Feltman told an emergency meeting of the Security Council.
The Israeli military said it had killed nearly 20 militants in exchanges of fire since the ground offensive started on Thursday night.
Gaza health officials said more than 50 Palestinians have been killed since then, including three young siblings from the Abu Musallam family who were killed when a tank shell hit their home.
At the morgue, 11-year-old Ahmed's face was blackened by soot, and he and his 14-year-old sister, Walaa, and 16-year-old brother, Mohammed, were wrapped in white burial shrouds. Their father, Ismail, said the three were sleeping when the shell struck and he had to dig them out from under the rubble.
Israel says it is going to great lengths to avoid civilian casualties and blames them on Hamas, accusing it of firing from within residential neighborhoods and using its civilians as "human shields." On Thursday, the U.N. refugee agency for Palestinians, UNRWA, said a routine check in one of its vacant Gaza schools found about 20 hidden rockets and called on militants to respect the "sanctity and integrity" of U.N. property.
Critics say it is the intense fire itself in such a densely populated area that leads to the deaths of innocent civilians. The U.N. children's agency, UNICEF, said at least 59 -- or one in five -- of the Palestinians killed children were under the age of 18. UNRWA said 40,000 Palestinians were seeking refuge in 34 of its shelters throughout the Gaza Strip.
Most countries have expressed support for Israel's right to defend itself, while urging it to minimize civilian deaths in its ground assault. President Barack Obama spoke with Netanyahu Friday and expressed his concern "about the risks of further escalation and the loss of more innocent life."
The operation also brought Israel its first military casualty. The circumstances behind the death of Staff Sgt. Eitan Barak, 20, were not made clear: Hamas's military wing said it ambushed Israeli units in the northern town of Beit Lahiya. A military spokesman said Barak was likely killed by friendly fire from a tank but it was not confirmed yet. The army said a number of soldiers were also wounded. Earlier in the week, an Israeli civilian died from Palestinian mortar fire and several others have been wounded.