Smoke rises from the northern Gaza Strip, background, as the Israeli Kibbutz of Kfar Azza is seen,... 
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"I don't want to disillusion anybody and residents of the south will go through difficult days," Barak said. "We do not seek war but we will not abandon our citizens to the ongoing Hamas attacks."
Israel called up tens of thousands of reservists in the event Palestinian militants in the West Bank or Hezbollah guerrillas in Lebanon decide to exploit the broad offensive in Gaza to launch attacks against Israel on other fronts.
The military said the country's north was on high alert in case Hezbollah guerillas decided to use its vast stockpiles of missiles against Israel. Israel and Hezbollah fought a 34-day war in the summer of 2006.
White House spokesman Gordon Johndroe said U.S. officials have been in regular contact with the Israelis as well as officials from countries in the region and Europe.
"We continue to make clear to them our concerns for civilians, as well as the humanitarian situation," Johndroe said.
The U.N. Security Council held emergency consultations Saturday night on the escalation in Gaza. Secretary-General Ban Ki-moon has urged key world leaders to intensify efforts to achieve an immediate truce including international monitors to enforce a truce and possibly to protect Palestinian civilians.
Israel's bruising air campaign against Gaza over the past eight days began days after a six-month truce expired. Gaza health officials say the air war has killed more than 480 Palestinians in an attempt to halt Hamas rocket attacks that were reaching farther into Israel than ever before. Four Israelis have been killed by rockets.
Israel is taking a risk by wading into intense urban warfare in densely populated Gaza that could exact a much higher toll on both sides and among civilians.
This sort of urban warfare has not gone well in past campaigns where Israel sent ground forces into Arab population centers in the Palestinian territories or in Lebanon wars in 1982 and 2006. Israeli forces have either gotten bogged down or sustained heavy casualties, without quelling violent groups or halting attacks for good.
The decision to expand the operation, while continuing to batter Gaza from the air and sea, was taken after Hamas refused to stop attacking Israel, government officials said. They spoke on condition of anonymity because discussions leading up to wartime decisions are confidential.
Before the ground incursion began, heavy Israeli artillery fire hit east of Gaza City, in locations where the military said Hamas fighters were deployed. The artillery shells were apparently intended to detonate Hamas explosive devices and mines planted along the border area before troops marched in.
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