There was no immediate claim of responsibility for the attack, which took place before dawn when no children were present.

However, extremists have accused the main UN aid agency here of corrupting Gaza's youth with its summer program of games, sports and human rights lessons for 250,000 children.

The UN's main competitor in running summer camps is the ruling Hamas movement, which says it reaches some 100,000 children. The Hamas camps teach Islam and military-style marching, along with swimming and horseback riding.

In a statement on Monday's attack, the UN said that about two dozen armed and masked men targeted a seaside day camp in central Gaza, one of dozens of UN sites across the Palestinian territory.

The assailants arrived at about 2:30 a.m., tied up an unarmed guard, then tried to set fire to two tents and a perimeter fence made from tarp-like material. Wielding knives, they slashed a plastic swimming pool, an inflatable slide and toys.

John Ging, the local UN chief, called it a "cowardly and despicable" attack and said the site would be rebuilt quickly. The about 2,000 children who attend the targeted facility joined another camp and will remain there until theirs is rebuilt, UN officials said.

Hamas police condemned the attack and said it was investigating.

Ging said the attack was the latest sign of growing levels of extremism in Gaza. He said it provides further evidence "of the urgency to change the circumstances on the ground that are generating such extremism."

Ging has repeatedly called for lifting of the 3-year-old border blockade of Gaza by Israel and Egypt, arguing the sanctions have mostly hurt ordinary Gazans, not Hamas. The blockade was imposed after the Hamas takeover of Gaza in 2007.

Last week, Israel said it would significantly ease the blockade, responding to international pressure following its deadly raid on a blockade-busting flotilla last month.

UN camps have been targeted by suspected extremists since their launch in 2007.

Last month, assailants vandalized one of the main camp sites, and last year a Hamas legislator waged a public campaign against the UN summer program. Gaza police said they have arrested five suspects in the previous camp attack, but would not give details about their affiliation.