Defense Minister Ehud Barak warned Thursday that militants in Hamas-ruled Gaza would pay a "heavy price" if they continued to target Israel and the military finalized preparations for a possible large-scale assault on the coastal territory.

Israel's military said four mortars were fired Thursday into Israel, causing no injuries. According to Army Radio, a fifth mortar landed at Israel's passenger crossing with Gaza as Gaza Christians were crossing en route to the West Bank town of Bethlehem for Christmas Day celebrations. Security officials had no additional information on that report.

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Also on Thursday, Foreign Minister Tzipi Livni was in Cairo to update Egyptian President Hosni Mubarak on Israel's objectives.

A day earlier, militants pummeled southern Israel with more than 80 rockets and mortars, causing no injuries but generating widespread panic. Israeli Cabinet ministers huddled for hours discussing Israel's response to the barrage and defense officials later told The Associated Press that ministers had approved a broad invasion of Gaza that would be launched after winter rains subsided.

"Whoever harms the citizens and soldiers of Israel will pay a heavy price," Barak warned.

Barak did not elaborate. But defense officials, speaking on condition on anonymity because they were not permitted to discuss the plans, said the Israeli operation would likely begin with surgical airstrikes against rocket launchers and continue with a land incursion.

Current weather conditions are hampering visibility and complicating air force missions, so the operation won't be launched until the skies clear, they said.

Israel has been reluctant to press ahead with a campaign liable to exact heavy casualties on both sides. Past incursions have not halted the barrages and officials fear anything short of a reoccupation of Gaza would fail to stop the rocket fire.

Israel left Gaza in 2005 after a 38-year occupation but still controls its border crossings. The Islamic Hamas militants seized control of Gaza in June 2007. The barrage Wednesday came days after a six-month truce expired and a day after Israeli troops killed three Palestinian militants along the Gaza-Israel border fence.

A Dialog survey published in the Haaretz daily Thursday indicated Israelis were slightly more opposed to an incursion that would put Israeli soldiers' lives at risk. The poll, which surveyed 475 people and had a margin of sampling error of 4.5 percentage points, found 40 percent of Israelis in favor of an operation and 46 percent opposed to it.

Livni's meeting with Mubarak on Israel's objectives was originally planned to attempt to renew the Egyptian-mediated truce. But after Wednesday's bombardment, Livni — who is running for prime minister in Israel's February elections — dismissed that option.

"There is a point where every country and every leadership says — and this is what we say tonight as well — enough is enough," she told a campaign rally.

The front pages of all Israeli newspapers Thursday were splashed with photos of panicked children running for cover. TV broadcasts showed buildings with gaping holes and traumatized people being removed from their demolished homes.

The military announced Wednesday night that it has hooked up an improved warning system against missiles — an indication of mounting concern that Gaza militants have expanded their range.

On Wednesday, Palestinian rockets reached as far as 12 miles from Gaza City.