JERUSALEM - Israeli soldiers and tanks fanned out along Gaza's borders Friday seeking out Hamas tunnel networks while pounding residential buildings with artillery and clashing with militants, as Prime Minister Benjamin Netanyahu warned of a possible "significant expansion" of Israel's ground offensive in the enclave.

The intensification of the conflict heightened concern about mounting Palestinian civilian casualties, even as the United States, Europe, and several influential Muslim countries expressed support for Israel's offensive to weaken the Islamist militant group Hamas.

In Washington, President Obama said he had spoken to Netanyahu earlier Friday and affirmed strong U.S. backing for Israel's right of self-defense. But Obama said he "also made clear that the United States and our friends and allies are deeply concerned about the risks of further escalation and the loss of more innocent life."

The president said Washington has been assured that the ground offensive was aimed at reducing the threat to Israel from Hamas-built tunnels on the border that are used to infiltrate Israel and stage attacks.

For now, Israeli forces pursued a modest operation Friday, moving roughly 1.5 miles into the Gaza Strip and zeroing in on farming areas and the outskirts of towns to search for tunnels, a senior Israeli intelligence official with knowledge of the ground incursion told reporters in a conference call.

"Our main target for now is to find, expose, and ruin as much as we can the offensive tunnels and continue to diminish, as much as we can, the launching of rockets," the Israeli intelligence official said, insisting on anonymity in accord with military protocol.

As many as 28 Palestinians have been killed since the ground operation began Thursday night, bringing the total Palestinian death toll to more than 260, with the injured topping 2,000, since the conflict erupted 11 days ago. The most recent fatalities included three children who perished in an air strike on an apartment complex in northern Gaza, according to the Gaza Health Ministry.

Hundreds of Palestinian families fled their houses, many carrying plastic bags filled with clothes and other possessions. About 47,000 Palestinians have sought refuge in U.N. shelters, more than half that number arriving in the first 24 hours of the ground offensive, the United Nations said late Friday.

Overnight, an Israeli soldier, 20, was killed in northern Gaza, underscoring risks to Israeli forces as they push into the densely populated ribbon of 1.7 million people.

He was the second Israeli killed in the conflict; on Tuesday in southern Israel, a civilian was killed near the Gaza border by a mortar round fired from the coastal territory.

"Because it is not possible to deal with the tunnels only from the air, our soldiers are now doing so on the ground," Netanyahu said Friday before entering a cabinet meeting. "We chose to commence this operation after we had exhausted the other possibilities, and with the understanding that without action, the price that we would pay would be much greater."

But Netanyahu also acknowledged that "there is no guarantee of 100 percent success" in the push to destroy the tunnels.

An expansion of the ground offensive, military analysts said, could entail a broadening of the mission to seek and destroy rocket launchers, weapons infrastructure, and storage facilities, and perhaps even eliminate key Hamas commanders and officials. Even as Israel has relentlessly bombarded Gaza, Hamas militants have succeeded in firing hundreds of rockets into southern and central Israel, rattling Israelis. As long as the militants possess rockets and tunnels, they remain a threat to Israel.

"If in the future we feel that airstrikes are not enough to reduce significantly the rocket launchers, I think there will be no other option but to use the ground forces to deal with this," said Shaul Shay, a retired Israeli military colonel and former deputy head of the Israel National Security Council.

Hamas has survived Israeli offensives in the past, including a major three-week ground operation in January 2009 and another weeklong air offensive in 2012, but in each case the militant group recovered. It now controls an arsenal of thousands of rockets, some long-range and powerful, and it has built a system of underground bunkers.

But Hamas is weaker than it was during the previous two offensives, with little global or even regional support from its main allies, Turkey and Qatar. Protests against the offensive took place Friday in Turkey, Jordan, and the West Bank.

In Israel, the government took additional precautions. Gatherings of more than 1,000 have been banned in Tel Aviv and Jerusalem, a decision that prompted the cancellation of an outdoor concert with Neil Young. Closer to the border, groups of 300 or more are now prohibited.

With Israeli forces and tanks poised to push deeper into Gaza, efforts to negotiate the Egyptian-initiated cease-fire between the warring sides were underway Friday.

Palestinian Authority President Mahmoud Abbas told reporters in Cairo that efforts to forge a lasting truce had been complicated by the ground incursion. But he said he had met with leaders of Islamic Jihad and "found a kind of acceptance of this initiative" and a willingness to persuade other Palestinian groups to accept it.

U.N. chief Ban Ki-moon was to leave Saturday for the Middle East to help mediate the Gaza conflict, Undersecretary General for Political Affairs Jeffrey Feltman said in New York, and French Foreign Minister Laurent Fabius was expected in Cairo to help promote a truce.

In Washington, Obama said Secretary of State John Kerry was ready to travel to the region to help facilitate a new cease-fire, "following additional consultations."


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