JERUSALEM -- Israeli ground forces backed by air and naval power pushed deeper into the Gaza Strip yesterday, engaging in fierce combat with Hamas fighters as they attempted to encircle the coastal region's largest city.

The second day of Israel's ground assault drew fresh international warnings that Gaza's humanitarian crisis would worsen in the coming days. The Palestinian death toll rose to roughly 507 with more than 2,000 wounded, including many civilians, according to United Nations and Palestinian health officials.

An Israeli soldier was fatally wounded in Gaza yesterday; three Israeli civilians and one soldier have also been killed by Hamas rocket and mortar fire inside Israel during the nine days of fighting. Israel insists its offensive is targeting only Hamas, the armed Islamist group that controls Gaza, and says the strip's 1.5 million residents do not face a humanitarian crisis.

Christopher Gunness, a U.N. spokesman, disagreed.

"We in the U.N., who have 9,000 or 10,000, workers on the ground, are in a much better situation to determine whether there is a humanitarian crisis, which there is, than Israel, which views Gaza through the lens of high-altitude bombers," he said.

Despite growing international criticism, the United States yesterday blocked passage of a U.N. Security Council statement urging an immediate cease-fire in Gaza. In Washington, meanwhile, Vice President Dick Cheney said yesterday that Israel did not seek U.S. approval before sending ground troops into Gaza.

"They didn't seek clearance or approval from us, certainly. They have said, now, for a period of months, ... [that] if the rocketing didn't stop, they felt they had no choice but to take action," Mr. Cheney said on CBS's "Face the Nation," adding that Israeli leaders "haven't told me exactly what they planned to do or when they plan to do it."

Sens. Harry Reid and Dick Durbin -- the top two Democrats in the Senate -- and Republican leader Mitch McConnell all described Israel's actions as understandable.

"I think what the Israelis are doing is very important," Mr. Reid said on NBC's "Meet the Press." "I think this terrorist organization, Hamas, has got to be put away. They've got to come to their senses."

Messrs. McConnell and Durbin spoke on "This Week" on ABC.

Israeli tanks and infantry units have punched into areas north and south of Gaza City, effectively dividing the strip in two, and by last night were also operating just east of the city, according to witnesses and Israeli military officials. Israeli forces seized control of open areas that Israeli military officials say Hamas uses to launch rockets into southern Israel. More than 500 rockets and mortars have landed in southern Israel over the nine days of fighting.

A senior Israeli military officer, speaking to foreign journalists in a conference call, said Israel was prepared to control those areas as long as needed to stop the rocket fire.

"We are not speaking about recapturing the Gaza Strip," said the official, who spoke on condition of anonymity. "This is not our objective. If we have to hold those areas, to stop the rockets, we will do this."

Israel so far has concentrated its ground forces in the northern half of the strip where hundreds of thousands of people live in a mix of refugee camps, crowded high-rise neighborhoods and agricultural villages. Meanwhile, Israeli jets and naval ships are bombarding the southern half of the strip, striking tunnels used to bring in weapons from the Egyptian Sinai and targeting Hamas leaders for assassination.

With a little more than a month until Israel holds national elections, pressure has been building for Israeli leaders to use the ground operation not just to weaken Hamas, but to paralyze it permanently.

"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," Israeli Prime Minister Ehud Olmert told his Cabinet yesterday in his first public comments on the ground offensive. "This operation was unavoidable."

Hamas triumphed in parliamentary elections in January 2006, winning day-to-day control of the Palestinian Authority. Amid rising political tension, Hamas seized control of Gaza in June 2007 from rival Fatah forces, causing a severe split in the Palestinian national movement. Israel has restricted food, medicine, and other supplies to the strip in an attempt to end Hamas rocket attacks. In its founding charter, Hamas is committed to Israel's destruction.

Israeli troops appeared to be drawing closer yesterday to engaging Hamas in densely populated urban areas that are the movement's historic strongholds. The Israeli soldier killed early yesterday afternoon was involved in "a heavy exchange of fire" during a battle close to the Jabalya refugee camp. Another Israeli soldier was severely wounded in the same incident, an Israeli military spokesman said.

It was Israel's first fatality of the ground invasion, which began Saturday night and followed a weeklong air assault on Gaza. Eighteen other Israeli soldiers were lightly wounded yesterday.

Inside Gaza, reports from local journalists and eyewitnesses described scenes of fear, destruction and worsening food shortages. Many Gazans barricaded themselves in their homes, while others risked fleeing on the open streets. More than 3,000 people have been displaced by the conflict and have sought haven in U.N. shelters.

In Gaza City, wounded Palestinians streamed into Shifa Hospital, including women and children, according to Palestinian health officials. Ambulances, sirens blaring, sped through the streets. People piled the injured into cars or carried them in their hands into the hospital.

Mr. Gunness, the U.N. spokesman, said fuel shortages are worsening and hospitals are running on emergency generators. Electricity is growing scarce. About 250,000 people are without potable drinking water, he said, also predicting that wheat flour would run out in three days.

"Gaza has been bifurcated by this offensive. We can't get food up to the north," Mr. Gunness said. "In anyone's book, this is a humanitarian crisis."