A war that began almost three weeks ago as an effort by Israel to stop Hamas rocket fire from killing Israeli civilians has been consumed by a bitter debate over who is responsible for the deaths of hundreds of Palestinian civilians in the Gaza Strip.

As the Palestinian death toll topped 1,000 on Wednesday, medical officials and aid organizations said civilians accounted for at least half of the total. An additional 4,700 Palestinians have been injured.

The Israeli military blames Hamas for using Gazans as human shields and for retreating to densely populated areas to fight the war. But Palestinians and human rights groups say that Israel has been reckless and that in pursuing Hamas, it has employed tactics that unnecessarily drive up the civilian toll.

A group of Israeli human rights organizations on Wednesday said Israel's behavior in Gaza represents "blatant violation of the laws of warfare and raises the suspicion, which we ask be investigated, of the commission of war crimes." Israel said it was doing everything it could to prevent civilian casualties. "We are taking many precautions, including dropping leaflets," said Maj. Avital Leibovich, an Israel Defense Forces spokeswoman. "Our pilots do U-turns when they see civilians near their targets."

The debate underscores the asymmetry of the conflict. On one side, Israel possesses a modern military, the strongest in the Middle East, with overwhelming firepower and the stated aim of sparing innocent Palestinian lives when it can, but above all protecting Israelis by crushing Hamas. On the other side, Hamas is an Islamist movement with a militia that has been badly damaged by the Israeli assault but that continues to fire rockets indiscriminately into Israeli civilian areas, as it has for eight years. It operates from within the civilian population in Gaza's densely packed cities and refugee camps.

Caught in between are the bulk of Gaza's 1.5 million residents, who are trapped in the narrow coastal strip with limited access to food, electricity and water as the war rages around them.

Since Israel launched a surprise aerial assault on Gaza on Dec. 27, under-resourced hospitals have struggled to keep up with the constant influx of severely injured patients, who arrive cradled in the arms of relatives or lying on makeshift stretchers.

Jawad Harb said that in the case of his 14-year-old cousin, there was no need to take him to the hospital. "He was standing on his balcony, watching the bombs. Then a big fragment penetrated his head," said Harb, who manages a project for the aid group CARE International that focuses on improving the lives of women in Gaza. "He died on the spot."

Later in the week, Harb stood in the street of the southern town of Rafah with his wife and six children and looked on as his neighborhood was systematically turned to rubble. His family had been warned to leave, he said, but in Gaza there is nowhere to go. The street seemed the safest place to ride out the airstrikes that landed like an earthquake every five minutes, each one sending a thick black plume into the sky from the spot of ground where a house once stood.

Some of the houses, Harb acknowledged, had concealed tunnels used by Hamas to smuggle weapons from Egypt. But most, he said, did not.

"They were destroying all the houses," said Harb, 44. "And if you stayed in your house, you died in your house."

Martha Myers, CARE's country director for the Palestinian territories, said Israel knows that deaths are inevitable when it unleashes massive amounts of firepower in a place as packed as Gaza, where about half the population is 16 or younger. "Gaza is a room crowded with children," she said.

The Geneva Conventions, an international agreement on a code of war, say civilians must be permitted to escape from fighting, but "the Gazans are not allowed to flee," Myers said. "That's a big problem." None of the border crossings is open. And no part of the strip -- which is about 25 miles long and five miles wide -- has been insulated from the fighting.

The absence of a way out of the strip has forced tens of thousands to flee their homes for other places within Gaza, according to U.N. officials. The United Nations has set up dozens of impromptu refugee centers in buildings normally used as schools. Even those, however, are not completely safe. Last week, Israeli forces shelled a school in the Jabalya refugee camp, killing 42 people. Doctors said most of the dead were women and children. Israel's military said it was responding to mortar fire from the building, an allegation the United Nations denied.

The International Committee of the Red Cross, meanwhile, has said Israel prevented its emergency workers from reaching dozens of injured and dead Palestinians for several days in the Zaytoun neighborhood south of Gaza City. Wounded and hungry children were subsequently found crying beside their dead mothers. Israel has said it is investigating.

Israel denies that it has deliberately obstructed the delivery of aid to civilians and accuses Hamas of doing so to exacerbate the civilian toll for propaganda purposes.

Israel says Hamas cares little for the well-being of its people. It puts children on the roofs of buildings that it knows are targets, and it prevents civilians from fleeing dangerous areas, the IDF says. It also booby-traps schools and attempts to draw Israeli forces into urban warfare that the group knows will result in high civilian body counts.

There are no craggy mountain caves or impenetrable jungles in which to hide in Gaza, nor are there functional military bases from which to stage attacks. The sparsely populated parts of the strip tend to consist of open farmland with little tree cover.

Before Israel sent in ground troops nearly two weeks ago, those areas were used by Hamas to fire rockets into southern Israel. But when Israeli troops rushed in, Hamas fell back to the winding alleys and concrete bunkers of Gaza's cities and camps. For guerrilla fighters, hiding in urban centers is the best way to stay alive.

"They basically believe that operating from civilian surroundings is helping to protect them because they know we are limited on attacking in civilian areas," said a senior Israeli intelligence official who spoke on the condition of anonymity. The official said Hamas leaders are hiding in hospitals and schools because they think Israel will not try to strike at them there.

They are also using urban areas to fire their rockets, which are unguided and, compared with the weapons in Israel's arsenal, crude. One landed in the Ashkelon home of Etty Ben Dayan on Monday. "The house was full of shattered glass," said Dayan, a 48-year-old mother of four who sews uniforms for Israel's soldiers. "The whole second floor was destroyed."

Her son was saved from injury or death when he made it into the home's bomb shelter seconds before the impact.

Hamas and its allies have continued to fire rockets into Israel at an average of 20 or more per day since Israeli ground troops moved into Gaza. But they have not exacted a high Israeli toll, as Hamas has said it hoped to do. Four Israelis have been killed by rocket and mortar fire from Gaza since the start of the war, three of them civilians. No one has been killed in Israel since the ground offensive began.

In the past eight years, Hamas has fired thousands of rockets into Israel. The pace picked up when Hamas, which won Palestinian legislative elections in 2006, ousted the rival Fatah party from Gaza in June 2007. Since then, Israel has imposed a crushing blockade on the strip in a bid to pressure Hamas to hold its fire.

Fred Abrahams, a senior researcher with the advocacy group Human Rights Watch, said Hamas is unquestionably committing a war crime when it fires rockets into civilian areas of Israel. But he said there are also serious questions over the legality of Israel's war tactics. "I am convinced the IDF has conceived its interpretation of the [international] law to come up to the very limit, and possibly over," he said.

One of the main problems, he said, is that the Israeli military has defined any Hamas-affiliated target as legitimate, not just military targets. In Gaza, where Hamas has been the ruling authority for 19 months and runs an extensive social services network, that definition has come to include government ministries, the parliament, the police academy, a university, mosques and a seaport.

A senior IDF commander acknowledged last week that Israel is not holding back. "We are very violent. We do not balk at any means to protect the lives of our soldiers," the commander said in a briefing for journalists.

The goal of targeting such a broad array of facilities, Israeli military officials say, is to break Hamas's will to continue firing rockets, not just its means. Israeli officials say they ultimately hope that Gazans become disgusted with Hamas and drive the group from power.

But Bassam Eid, executive director of the Palestinian Human Rights Monitoring Group, doubts that can work. Whether or not Hamas is intentionally raising the Palestinian death toll to elicit sympathy, he said, the killing of so many civilians has helped the group's standing among Palestinians and across the Arab world.

"It's not in the interest of the Israelis. It's not in the interest of Palestinian society," Eid said. "But it's in the interest of Hamas."