THOUSANDS of Israeli troops, backed by columns of tanks, poured into Gaza after nightfall yesterday to do battle with Palestinian militants as Israel launched a much-anticipated ground offensive in a widening war on Gaza's Hamas rulers.

Israeli leaders said the operation, meant to quell militant rocket and mortar fire on southern Israel, would not end quickly, but that the objective was not to reoccupy Gaza or topple Hamas. The depth and intensity will depend in part on parallel diplomatic efforts that so far haven't yielded a truce proposal acceptable to Israel, the officials said.

Israel mounted a bruising air campaign against Gaza eight days ago, striking hundreds of targets and killing some 500 Palestinians. The decision to expand the operation to a ground push, while continuing to batter Gaza from the air and sea, was taken after Hamas refused to stop attacking Israel, government officials said.

About 2 1/2 hours after ground troops moved in, Israeli Defense Minister Ehud Barak went on national TV to tell Israelis that the expanded campaign against Hamas "will not be easy and it will not be short."

"We do not seek war but we will not abandon our citizens to the ongoing Hamas attacks," Barak said.

Hamas warned Israel that Gaza "would become a graveyard for you."

At the same time, Israel called up tens of thousands of reservists in the event Palestinian militants in the West Bank or Hezbollah guerrillas in Lebanon should decide to exploit the broad offensive in Gaza to launch attacks against Israel on other fronts.

The night sky over Gaza was lit by the flash of bullets and balls of fire from tank shells. Sounds of explosions were heard across Gaza City, the territory's biggest city, and high-rise buildings shook from the bigger booms.

Israel also kept pounding Gaza with airstrikes, with F-16 warplanes hitting three targets within a few minutes, including a main Hamas security compound.

Palestinian health officials said civilians were among the dead, including a woman, her son and her father who died after a shell hit their house.

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 rolled in.

Witnesses in Gaza said that in the first phase, Israeli ground forces had moved several hundred meters inside Gaza. Israeli security officials said initial clashes with militants took place in open fields and soldiers did not immediately move into Gaza's crowded cities, where warfare would likely get much deadlier.

In the initial, airborne phase of Israel's onslaught, militants were not deterred from bombarding southern Israel with more than 400 rockets -- including dozens that extended deeper into Israel than ever before. Four Israelis were killed by rockets in that first phase.

The launch of the ground operation did not make Hamas any less defiant.

"You entered like rats," Hamas spokesman Ismail Radwan told Israeli soldiers in a statement on Hamas' Al Aqsa TV, broadcast shortly after the start of the invasion.

"Your entry to Gaza won't be easy. Gaza will be a graveyard for you, God willing," he said.

Militants fired six rockets into Israel in the first few hours after the ground push began, the military said.

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.

"Their ground action is part of their overall operation," Johndroe said. "We continue to make clear to them our concerns for civilians, as well as the humanitarian situation."

The UN Security Council scheduled emergency consultations yesterday night to address the escalation of violence in Gaza. Secretary-General Ban Ki-moon has urged key world leaders to intensify efforts to achieve an immediate Israeli-Hamas truce that includes international monitors to enforce a truce and possibly to protect Palestinian civilians.

Gunbattles could be heard as Israeli troops crossed the border. Local TV networks broadcast images of troops with camouflage face paint marching single file. The troops were also backed by helicopter gunships.

Gaza is an impoverished territory of 1.4 million people who have been confined to their tiny coastal strip since Hamas violently seized power in June 2007, provoking an Israeli and Egyptian blockade. Israel maintained control of its border crossings with Gaza even after it pulled all of its settlers and soldiers out of the territory in late 2005, ending a 38-year occupation.

Hamas began to emerge as Gaza's main power broker when it won Palestinian parliamentary elections three years ago.

Israeli Prime Minister Ehud Olmert said his government decided to mount a land operation against Hamas in Gaza despite the risk it posed to thousands of soldiers.

"I want to be able to go to the Israeli public and all the mothers and say, we did everything in a responsible manner," Olmert said in a statement released by his office. "In the end, we reached the moment where I had to decide to send out soldiers."

Israel has a long and largely unsuccessful history of sending ground forces into Arab population centers. Israeli forces have either gotten bogged down or sustained heavy casualties, without quelling violent groups or halting attacks for good.

Top Israeli Cabinet ministers met with leading security officials for four hours yesterday before deciding to authorize the ground invasion. Olmert told the meeting that Israel's objective was to bring quiet to southern Israel but "we don't want to topple Hamas," a government official quoted the prime minister as saying. The official spoke on condition of anonymity because he was not supposed to share the information.

The immediate aim of the ground operations was to take control of sites militants use as rocket-launching pads, the military said. It added that a large number of troops were taking part, without specifying how many, suggesting the ground operation could expand in other directions.

It said the forces included infantry, as well as engineering, artillery and intelligence units.

The military announced that it had begun to mobilize tens of thousands of reserve soldiers in addition to the 10,000 soldiers that defense officials have said were already massed along the border. The military also said the country's north was on high alert in case Hezbollah guerillas decided to use their vast stockpiles of missiles against Israel.

Israel rebuffed a French truce proposal earlier this week, and yesterday, France denounced the Israeli ground operation as well as the Palestinian rocket fire. Egypt and the Arab League both condemned the operation as well.

"This aggression comes as a loud challenge to the international will, which has been demanding for a whole week that Israel halt the operations," Egyptian Foreign Minister Ahmed Aboul Gheit said.

Aboul Gheit said Egypt would continue its contacts with all the concerned parties to try to reach a cease-fire. Israel's aerial bombardment of Gaza began just days after a six-month truce brokered by Egypt expired after unraveling for weeks.