Jerusalem  - Israel's ban on allowing building materials into Gaza is hindering reconstruction work on the tens of thousands of homes destroyed in the Israeli offensive, the United Nations Relief and Works Agency said Wednesday.

"The list is long and the need is urgent but the total ban on building supplies remains in force," UNRWA Spokesman Christopher Gunness told a news conference in Jerusalem, held to mark two years since Israel imposed a blockade on the Gaza Strip.

Some 52,400 houses have been destroyed in the enclave, affecting 250,000 people, Gunness said. In addition, 800 industrial properties were damaged or destroyed in the Israeli offensive from December 28 to January 18, while 14 mosques were destroyed, as were 10 schools, with a further 240 schools damaged.

Israel imposed the blockade after the militant Hamas movement assumed full security control of the Gaza Strip amid an intensification of rocket attacks from the enclave on southern Israeli towns and villages.

Since the siege was instituted, Israel has allowed only basic good to enter the salient.

"We are receiving basic stuff but ... we need more than the basics and it is not being provided," Gunness said. "On a nutritional level we are covering two-thirds of the needs."

He said that in addition, 400,000 people were drinking contaminated water.

Michael Bailey of the Oxfam charity, told the news conference that 80 per cent of the inhabitants of the Strip depended on food assistance.

He charged that since November 2008 no petrol or diesel, save for that provided by UNRWA, has been allowed into the Gaza Strip, with the result that the Gaza power plant receives only 70 per cent of weekly fuel requirements. Only 50 per cent of the cooking gas requirements were permitted to be shipped into the Strip, he added.

"This is a collective punishment not only in terms of food access, but also for the development of the agriculture," he said.

"The blockade must be lifted immediately," Eva Tomic, of the office of the UN Human Rights Commissioner, told the news conference.(dpa)