GAZA CITY, Gaza Strip (AP) — Israel withdrew most of its ground troops from the Gaza Strip on Sunday in an apparent winding down of the nearly monthlong operation against Hamas that has left more than 1,800 Palestinians and 60 Israelis dead.

Even as Israel said it was close to completing its mission, heavy fighting raged in parts of Gaza, with at least 10 Palestinians killed in an alleged Israeli airstrike near a U.N. shelter, according to U.N. and Palestinian officials. The U.S. and the United Nations condemned the attack in unusually strong terms.

And with Hamas officials vowing to continue their fight, it remained uncertain whether Israel could unilaterally end the war.

Israel launched its military operation in Gaza on July 8 in response to weeks of heavy rocket fire, carrying out hundreds of airstrikes across the crowded seaside territory. It then sent in ground forces July 17 in what it said was a mission to destroy the tunnels used by Hamas to carry out attacks.

Hamas has fired more than 3,000 rockets into Israel during what has turned into the bloodiest round of fighting ever between the two enemies.

Lt. Col. Peter Lerner, an Israeli military spokesman, confirmed the bulk of ground troops had been pulled out of Gaza after the military concluded it had destroyed most of the tunnel network.

He said Israel had detected some 30 tunnels that were dug along the border for what he called a "synchronized attack" on Israel.

"We've caused substantial damage to this network to an extent where we've basically taken this huge threat and made it minimal," he said. The army had thousands of troops in Gaza at the height of the operation.

In southern Israel, armored vehicles could be seen rolling slowly onto the back of large flatbed trucks near the Gaza border, while soldiers folded flags from atop a tank and rolled up their belongings and sleeping bags.

Lerner said, however, that the operation was not over and that Israel would continue to target Hamas' rocket-firing capabilities and its ability to infiltrate Israel.

While Israeli Prime Minister Benjamin Netanyahu has vowed to press on against Hamas, he is coming under international pressure to halt the fighting because of the heavy civilian death toll.

U.N. officials say more than three-quarters of the dead have been civilians, including the 10 people killed Sunday at a U.N. school that has been converted into a shelter in the southern town of Rafah.

U.N. Secretary-General Ban Ki-moon called the attack a "moral outrage and a criminal act" and demanded a quick investigation, while the U.S. State Department said Washington was "appalled" by the "disgraceful" attack.

According to witnesses, Israeli strikes hit just outside the main gates of the school. The Red Crescent, a charity, said the attack occurred while people were in line to get food from aid workers. Gaza health official Ashraf al-Kidra said in addition to the dead, 35 people were wounded.

Robert Turner, director of operations for the U.N. Palestinian refugee agency in Gaza, said the building had been providing shelter for some 3,000 people. He said the strike killed at least one U.N. staffer.

"The locations of all these installations have been passed to the Israeli military multiple times," Turner said. "They know where these shelters are. How this continues to happen, I have no idea."

Inside the U.N. school's compound, several bodies, among them children, were strewn across the ground in puddles of blood. "Our trust and our fate are only in the hands of God!" one woman cried.

The Israeli military said it had targeted three wanted militants on a motorcycle in the vicinity and was "reviewing the consequences of this strike."

In the current round of fighting, U.N. shelters have been struck by fire seven times. UNRWA, the U.N. agency that assists Palestinian refugees, says Israel has been the source of fire in all instances. But it also has said it found caches of rockets in vacant UNRWA schools three times.

Israel accuses Hamas of using civilian areas for cover and says the Islamic militant group is responsible for the heavy death toll because it has been using civilians as "human shields."

Israeli artillery shells slammed into two high-rise office buildings Sunday in downtown Gaza City, police and witnesses said. Al-Kidra said more than 50 Palestinians were killed, including 10 members of one family in a single strike in the southern Gaza Strip.

Israel said that it attacked 63 sites on Sunday and that nearly 100 rockets and mortars were fired at Israel.

Israeli officials said the military would reduce its ground activities in Gaza but would respond to continued attacks from Gaza with airstrikes.

"It's not a withdrawal," said Israeli Cabinet minister Amir Peretz told Channel 10 TV. "It's setting up a new line that is a more controlled line with the air force doing its work."

In Gaza, Hamas officials said they would not halt the rocket fire without an end to an Israeli blockade of the territory that has devastated the local economy. Israel imposed the blockade in 2007, saying the measures are needed to keep Hamas from arming.

"If Israel stops unilaterally, Hamas will declare victory and will not grant any security or truce to Israel," said one senior official, who spoke on condition of anonymity because he was discussing internal Hamas deliberations. "In this case, we are going to live under a war of attrition until a political solution is found."

In Cairo, Egyptian and Palestinian negotiators held talks over a potential cease-fire. After accusing Hamas of repeatedly violating humanitarian cease-fire arrangements, Israel said it would not attend the talks and there was "no point" in negotiating with the militant group.

Meanwhile, the Israeli military death toll rose to 64 after Israel announced that Hadar Goldin, a 23-year-old infantry lieutenant feared captured in Gaza, was actually killed in battle. Some 15,000 people attended his funeral Sunday.

Israeli Defense Minister Moshe Yaalon revealed on his Facebook page Sunday that he is a distant relative of Goldin and had known him his whole life. The information was previously kept under wraps while Goldin was feared abducted.

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Federman reported from Jerusalem. Yousur Alhlou in Jerusalem, Ibrahim Barzak in Gaza City, Mohammed Daraghmeh in Ramallah, West Bank, and Maggie Michael in Cairo contributed reporting.