Armed assailants in black masks burned and vandalized a U.N. summer camp site Sunday and left behind three bullets next to written death threats against U.N. officials — the latest escalation of tensions between Islamic extremists and U.N. representatives in Hamas-ruled Gaza.

Also Sunday, a U.N. 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 U.N. camp site on the Gaza shore, near Gaza City.

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

The U.N. 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 U.N. aid agency and who was among three U.N. 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.

The guard said the vandals then tied his hands, slashed or burned eight tents and set fire to 30 large plastic water tanks and other facilities.

Ging said the damage would be repaired and that all 143 camps would open June 12, as scheduled.

The Hamas government denounced the attack.

In the past, shadowy extremist groups, some with ties to Hamas, have attacked Internet cafes, music stores and other sites they felt had a corrupting influence on Gazans.

The U.N. 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 U.N. 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.

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 U.N. figures.

The blockade, imposed by Israel and Egypt after the Hamas takeover of Gaza, has significantly hampered rebuilding. Israel only allows what it defines as "humanitarian goods" into Gaza, including foods and medicines. Most construction materials, such as steel and cement, are banned because Israel argues they could be used by Hamas for military purposes.

The Israeli military office in charge of activity in Gaza responded by reiterating its policy statement about the blockade, listing humanitarian items allowed in but not referring to building supplies.

Also Sunday, Israel announced that Egyptian intelligence chief Omar Suleiman will meet Monday with Israeli officials about regional security issues.

In Jerusalem, an Israeli Cabinet committee gave preliminary approval to a bill to strip Hamas prisoners held by Israel of many privileges until a captured Israeli soldier is freed. The Hamas prisoners would lose newspapers, TVs and other perks. Palestinians captured the soldier, Sgt. Gilad Schalit, in 2006. They have allowed him to send messages only a few times and have barred Red Cross access.