GAZA CITY - The Hamas rulers of Gaza on Tuesday said they had arrested a group of suspects over the torching of a UN children's summer camp, in which attackers also threatened the head of a UN agency.

"A group of people suspected of involvement in the damage caused to tents in the UNRWA holiday camp was arrested," the Hamas interior ministry said in a statement.

About 20 men, some carrying assault rifles, tear up large plastic tents and burn storage facilities two days after unknown group accuses UN of promoting immorality in religiously conservative enclave. Hamas government condemns attack.

"They are being investigated," it said.

Masked gunmen on Sunday stormed a camp run by the UN agency for Palestinian refugees, and set it on fire.

They also left a letter threatening "harsh measures" against the Gaza director of UNRWA, John Ging.

Dozens of attackers assaulted a guard, tied him up and left the threatening letter with four bullets, the agency said.

A large tent and a few plastic swimming pools were damaged.

The seaside camp was one of several set up across Gaza to teach swimming, sports and other activities to some 250,000 schoolchildren during summer vacation and was to open in the middle of next month.

No one claimed the attack but it appeared to have been carried out by Islamic extremists who have attacked Western symbols in Gaza in the past and who are opposed to the camps because they allow boys and girls to mingle.

Ging later told reporters the attack was carried out by people with an "extremist mentality" and vowed to continue his agency's work in the Gaza Strip.

Hamas and other Palestinian factions strongly condemned the attack.

An Islamist movement, Hamas has done little to impose Islamic law in Gaza, but smaller and more radical groups have carried out a string of bombings of beauty parlours, coffee shops, Christian institutions and Internet cafes.

UNRWA provides food aid, education and health services to more than one million registered Palestinian refugees in Gaza -- two thirds of the population of the coastal strip.

It has been a key source of humanitarian aid to Gaza since Israel and Egypt tightened border closures in the wake of the Hamas movement's violent takeover of the territory in June 2007.

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