A group of armed men has vandalised a UN summer camp in the Gaza Strip, the second such attack in just over one month.

The attackers destroyed tents, toys, a plastic swimming pool and set much of the camp's equipment on fire on Monday.

John Ging, the head of the UN Relief and Works Agency (UNRWA) in Gaza, called it a "cowardly act," and vowed to rebuild the camp quickly.

"This is another example of the growing levels of extremism in Gaza, and further evidence, if that were needed, of the urgency to change the situation on the ground," Ging said.

The United Nations runs a network of summer camps for some 250,000 children in Gaza, which include games, sports and human rights lessons.

Hamas, which controls the Gaza Strip, is UNRWA's largest competitor, running a network of camps for roughly 100,000 children.

Groups with "misguided ideas"

There was no immediate claim of responsibility for Monday's attack.

Hamas condemned the raid and vowed to investigate. In a statement, the interior ministry blamed the attack on "groups led by those with misguided ideas who want to distort the situation in Gaza".

The Popular Front for the Liberation of Palestine also issued a statement condemning the attack.

The previous camp attack happened on May 23, when dozens of masked men tied up a night watchman, set fire to equipment and left a threatening letter for Ging.

The attacks have been widely attributed to extremist groups in Gaza that view the camps as religiously unacceptable, because boys and girls mingle freely.