ISRAELI ground troops and tanks cut swaths through the Gaza Strip early today, bisecting the coastal territory and surrounding its biggest city as the new phase of a devastating offensive against Hamas gained momentum.

Thousands of soldiers in three brigade-size formations pushed into Gaza after nightfall Yesterday, beginning a long-awaited ground offensive after a week of intense aerial bombardment. Black smoke billowed over Gaza City at first light as bursts of machine gun fire rang out.

TV footage showed Israeli troops with night-vision goggles and camouflage face paint marching in single file. Artillery barrages preceded their advance, and they moved through fields and orchards following bomb-sniffing dogs ensuring their routes had not been booby-trapped.

The military said troops killed or wounded dozens of militant fighters, but Palestinian medical teams in Gaza, unable to move because of the fighting, could not provide casualty figures. Hamas said only three of its fighters had been killed, and Gaza health officials said eight civilians also died, including a 12-year-old girl in the northern Gaza town of Beit Lahiya and four family members killed in an airstrike in southern Gaza.

Palestinian officials say nearly 480 people, including dozens of civilians, were killed in the weeklong air offensive.

Army ambulances were seen bringing Israeli wounded to a hospital in the southern Israeli city of Beersheba. The military reported 30 Israeli troops were wounded, two seriously, in the opening hours of the offensive.

Defense Minister Ehud Barak predicted a long and difficult campaign in Gaza, a densely populated territory of 1.4 million where militants operate and easily hide among the crowded urban landscape.

The war will "not be short and it will not be easy," Barak said in a nationally televised address late Yesterday. "We do not seek war but we will not abandon our citizens to the ongoing Hamas attacks."

Hamas threatened to turn Gaza into a "graveyard" for Israeli forces.

"You entered like rats," Hamas spokesman Ismail Radwan told Israeli soldiers in a statement on Hamas' Al Aqsa TV. "Gaza will be a graveyard for you, God willing," he said.

The ground operation is the second phase in an offensive that began as a weeklong aerial onslaught aimed at halting Hamas rocket fire that has reached deeper and deeper into Israel, threatening major cities and one-eighth of Israel's population.

Rocket fire has persisted, however, and several rockets fell in Israel on Sunday morning, causing no casualties. In much of southern Israel school has been canceled and life has been largely paralyzed.

While the air offensive presented little risk for Israel's army, sending in ground troops is a much more dangerous proposition. Hamas is believed to have some 20,000 gunman who know the dense urban landscape intimately. For months, Israeli leaders had resisted a ground invasion, fearing heavy casualties.

Israeli Prime Minister Ehud Olmert said he was aware of such risks, but decided that the government had no more choice.

"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."

He stressed the campaign's objective is to restore quiet to Israel's south, not to topple Hamas or reoccupy Gaza. Israel considers Hamas, which has controlled Gaza since June 2007 and is sworn to Israel's destruction, a terrorist group.

Israel has launched at least two other large ground offensives in Gaza since withdrawing its troops from the area in 2005. But the size of this latest operation dwarfs those previous attempts, with at least three times the firepower.

Israel also has called up tens of thousands of reserve soldiers, which defense officials said could enable a far broader ground offensive as the operation's third phase. The troops could also be used in the event Palestinian militants in the West Bank or Hezbollah guerrillas in Lebanon decide to launch attacks. Hezbollah opened a war against Israel in 2006 when it was in the midst of a large operation in Gaza.

The officials spoke on condition of anonymity because the military's preparations are classified.

An armored force south of Gaza City penetrated as deep as the abandoned settlement of Netzarim, which Israel left along with other Israeli communities when it pulled out of Gaza in 2005, both military officials and Palestinian witnesses said.

That move effectively cut off Gaza City, the territory's largest population center, from the rest of Gaza to the south.

The offensive focused on northern Gaza, where most of the rockets are fired into Israel, but at least one incursion was reported in the southern part of the strip. Hamas uses smuggling tunnels along the southern border with Egypt to bring in weapons.

Warplanes struck about four dozen targets overnight, including tunnels, weapons storage facilities, areas used to launch mortars and squads of Hamas fighters, the military said.

Gunboats backed up the ground forces, attacking Hamas intelligence headquarters in Gaza City, rocket-launching areas and positions of Hamas marine forces.

Hamas was responding with mortar shells and rocket-propelled grenades. Field commanders communicated over walkie talkie, updating gunmen on the location of Israeli forces. Commanders told gunmen in the streets not to gather in big groups and not to use cell phones. Hamas' TV and radio stations, broadcasting from secret locations after their offices were destroyed, remained on the air, broadcasting live coverage.

Ground forces had not entered major Gaza towns and cities by early this morning, instead fighting in rural communities and open areas militants often use to launch rockets and mortar rounds. Militants also fire from heavily populated neighborhoods.

Residents of the small northern Gaza community of al-Attatra said soldiers moved from house to house by blowing holes through walls. Most of the houses were unoccupied, their residents already having fled.

Israel launched the air campaign against Gaza on Dec. 27. Gaza health officials say more than 480 Palestinians were killed in the first eight days of the operation. The breakdown of combatants and civilians remains unclear, but the UN says at least 100 civilians were killed in the initial, aerial phase of the war.

Hundreds of rockets have hit Israel so far, and four Israelis have been killed.