Also Sunday, a UN agency reported that three-quarters of the damage inflicted on Gaza by Israel's war against Hamas more than a year ago has not been repaired or rebuilt. The report warned that the international community is being increasingly sidelined in Gaza because of Israel's blockade of the territory.

There was no immediate claim of responsibility for the attack on the UN camp site on the Gaza shore, near Gaza City.

However, Islamic groups, including Hamas, have repeatedly railed against the UN as being a bridgehead for the West and corrupting Gaza's youth by teaching "un-Islamic" subjects in schools and summer camps.

The UN summer program offers sports, crafts and other activities to some 250,000 Gaza children and teenagers.

Hamas, which seized Gaza by force three years ago, has set up rival camps with a heavy emphasis on Islamic teachings.

"Vandalism linked to extremism, that encapsulates what we are dealing with here," said John Ging, who heads Gaza's largest UN aid agency and who was among three UN officials the assailants threatened to kill unless the summer camps are canceled.

About 30 assailants in black uniforms and masks arrived at the camp site in jeeps at about 2:30 a.m., said the night watchman, Ibrahim Alawi, adding that the attackers closed off access roads.

The guard said the vandals then tied his hands, slashed or burned eight tents and set fire to 30 large plastic water tanks. Ging said the attackers also vandalized toilets, showers and storage facilities.
 
HAMAS DENOUNCES ATTACK

The Hamas government denounced the attack and said it would bring the perpetrators to justice.

In the past, shadowy groups have attacked Internet cafes, music stores and other sites they felt had a corrupting influence on young Gazans.

Ging said the damage would be repaired and that all camps would open June 12, as scheduled. In all, there are 143 camp sites, UN officials said.

"We won't be intimidated," Ging said. "We draw our strength and confidence from the silent majority (of Gazans). Statements and acts from an extremist minority will not alter our course."

The UN report on war reconstruction, meanwhile, found that Gazans have managed to carry out small-scale repairs with recycled rubble and material smuggled through tunnels under the border with Egypt. But thousands of homes, businesses and schools have not been reconstructed or repaired, said the UN Development Program, citing Israel's three-year border blockade.

The findings show that the international community has been largely ineffective in addressing the needs of Gazans, said the top UNDP representative in the region, Jens Toyberg-Frandzen.

"In view of the scale of the needs, international assistance in Gaza is tantamount to tinkering on the edges," he wrote. "Maintaining the status quo will not enable Gaza to recover, and certainly not with the support of the international community, irrespective of intentions."

Israel launched a three-week military offensive against Gaza's Hamas rulers in December 2008, in an attempt to halt years of rocket fire from Gaza on Israeli border towns. The war killed about 1,400 Gazans, along with 13 Israelis. More than 6,200 homes, hundreds of businesses, as well as dozens of schools and clinics, were destroyed or damaged, according to UN figures.