Masked Palestinian gunmen set fire to a U.N.-run summer camp in Gaza on Monday, officials and witnesses said, the second time in recent weeks that militants have attacked a United Nations Relief and Works Agency (UNRWA) children's camp in the Hamas-controlled territory.

About 25 militants stormed the recreation facility and assaulted security guards before setting the building on fire, U.N. spokesman Adnan Abu Hasna said.

In May, gunmen set a building on fire after accusing the United Nations of promoting immorality in the Islamist-ruled enclave.

John Ging, UNRWA's director of operations in the Gaza Strip, said Monday's attack was a "cowardly and despicable act".

"UNRWA will rebuild the camp immediately and will continue with its Summer Games programme which is so important for the physical and psychological well-being of Gaza's children, so many of whom are stressed and traumatised by their circumstances and experiences," he said in a statement.

Hamas officials had no immediate comment.

Fundamentalist Muslims, or Salafis -- whose agenda of global or holy war against the West conflicts with Hamas's nationalist goals -- have stepped up attacks in the Gaza Strip over the past several months, targeting Hamas security men and offices.

Hamas itself has also clamped down on behaviour and events it views as immoral and in April sent its security forces to break up Gaza's first major hip-hop concert.

The Islamist group wrested control of the Gaza Strip from Palestinian President Mahmoud Abbas's secular Fatah movement in fighting in 2007. Israel, together with Egypt, tightened its blockade on the territory after Hamas took over.