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Israeli troops clashed with Hamas militants Sunday, leading to a number of casualties on both sides as soldiers, tanks and helicopter gunships pushed deeper into the Gaza Strip.

While Israel said Hamas was suffering punishing blows, growing concern over civilian casualties and the growing threat of a humanitarian crisis in Gaza has sparked widespread international outrage over the widening invasion.

After moving into Gaza after nightfall, Israeli tanks and troops bisected the narrow strip of land and surrounded Gaza City.

The Israeli military says that soldiers have killed or wounded dozens of militants, but according to Gaza officials, at least 31 civilians have been killed in the ground offensive.

Since Israel launched its air offensive on Dec. 27, 512 people in Gaza have been killed, The Associated Press reported Sunday night.

Palestinian and UN officials say at least 100 of those casualties have been civilians.

However, Palestinian medical officials could not confirm the exact number of casualties, as they are unable to move around Gaza due to the fighting. According to Hamas, four of its fighters have been killed.

The Israeli military reports that one soldier has been killed and 30 wounded, two seriously. Television news images showed army ambulances bringing soldiers to a hospital in the southern Israeli city of Beersheba.

Israeli Prime Minister Ehud Olmert said Sunday that Israel would protect its civilians from rocket attacks, which continued throughout the air assault.

"This morning I can look every one of you in the eyes and say the government did everything before deciding to go ahead with the operation," Olmert said.

Israeli officials have said they do not intend to occupy the Gaza Strip and have been clear that the goal is to stop rocket attacks.

However, the ground offensive would not be "a rapid one that would end in hours or a few days," warned one senior military officer, who asked not be named.

Palestinian President Mahmoud Abbas called the ground offensive "brutal aggression," and reached out to his rivals in Hamas, which controls the Gaza Strip.

In response to the troop movement, Hamas officials said Gaza would become a "graveyard" for Israeli soldiers.

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

Offensive widens

The Israeli army has called up tens of thousands of reserve soldiers in advance of a potential third phase that could include a wider ground offensive.

Defence officials, speaking on condition of anonymity, said the troops could be called in should militants in the West Bank or Hezbollah fighters in Lebanon launch attacks of their own.

The invasion could also spark a bloody, prolonged war pitting Israeli soldiers against Hamas fighters in winding streets, alleys and apartment hallways. Hamas is believed to have about 20,000 armed fighters and has been preparing for an Israeli invasion.

But early Sunday, the Israeli offensive targeted the northern Gaza Strip, cutting off Gaza City's 400,000 residents from the rest of the region to the south.

It is from northern Gaza that most rockets are fired into Israel.

In an interview Sunday morning on Newsnet, Â鶹ӰÊÓ correspondent Janis Mackey Frayer said that partitioning Gaza is an attempt by the Israeli army to "impede the movement of Hamas and other militant groups. They believe it disables fighters from being able to move around, it disables rocket squads, it's trying to effectively take control of movement within the Gaza Strip."

The heaviest fighting Sunday occurred in the town of Beit Lahiya, where an artillery shell killed eight civilians as they ran to a nearby school to take cover, according to local paramedics.

Earlier this week, Beit Lahiya was hit by at least two air strikes, which also killed civilians.

Israeli troops were also reportedly searching houses in the northern town of al-Attatra. However, many residents had already fled in advance of the soldiers' arrival.

Humanitarian disaster looms

The week-long air offensive has knocked out power to many homes and businesses in the Gaza Strip, leaving many families without heat and many shops unable to store food supplies.

"They're traumatized, they're terrorized and they're trapped," John Ging of the UN Relief and Work Agency, which has 10,000 Palestinian volunteers in Gaza, told Â鶹ӰÊÓnet on Sunday. "The population is without water, food is critically low, there's no electricity."

Ging also said that the dead and injured have left Gaza hospitals "completely and utterly overburdened."

According to Ging, one Palestinian UN aid worker volunteering at a local hospital has been killed. He did not elaborate on exactly how the worker died.

The Israeli army dropped leaflets on Gaza Saturday, warning residents to take cover in advance of the ground offensive.

"The problem of course is that Gaza is a sealed place. It is confined, it is densely populated," Mackey Frayer said.

French President Nicolas Sarkozy will visit the region Monday in an attempt to broker a cease fire and encourage the resumption of peace talks.

International response

The widespread destruction in Gaza has drawn criticism from many global leaders, including UN Secretary General Ban Ki-moon, who expressed "extreme concern and disappointment" over the operation.

The French government also denounced the invasion, and Muslim nations like Egypt, Turkey and Jordan condemned what they called Israeli aggression.

However, the U.S. has fully blamed Hamas for the fighting, and on Saturday night, American officials blocked a UN statement calling for a ceasefire because they believed it didn't adequately address Hamas rocket fire.

With files from The Associated Press