Palestinian workers will need one year to clear a half million tonnes of concrete rubble from Gaza Strip districts bombed and bulldozed by Israeli forces during their winter offensive, the United Nations said on Thursday.

The United Nations Development Programme was beginning its rubble removal project six months after the 3-week war ended on Jan. 18, with still no idea of when organised reconstruction could begin, said the UNDP's Jens-Anders Toyberg-Frandzen.

"At the moment we cannot rebuild. That is of course very sad. We don't have access to cement, we don't have access to construction material because of the borders being closed. So we cannot build houses," he said.

Thousands of buildings were destroyed during Israel's Operation Cast Lead, launched on Dec. 27 with the declared aim of forcing Islamist Hamas fighters and other Palestinian groups to stop firing rockets and mortars at Israeli towns.

A Palestinian rights group says 1,417 people were killed, 926 of them civilians. The Israeli army put the death toll at 1,166 and estimated 295 were civilians. Israel says 13 of its citizens, 10 of them soldiers, were killed in the 22 days.

Whole districts were razed during the operation to minimise the risk of Israeli casualties from guerrilla small-arms attacks and booby-trap bombs, and to open up fields of fire for Israeli tanks, artillery and armoured infantry units in Gaza.

Israel prohibits the import of cement and steel reinforcing rods on the grounds that they could be used for military purposes by Hamas, such as constructing defences. But these are also the materials the people of Gaza build their homes with.

"The U.N is constantly advocating for opening of the borders so that material can come in for humanitarian purposes, and also to be able to ensure or help the Palestinians in Gaza get a reasonable and decent life again," the UNDP representative said.

"But we haven't succeeded so far."

The rubble -- estimated to total 600,000 tonnes according to the UNDP -- is to be collected at a central dump where it will later be crushed and used for new construction -- whenever that can begin in earnest.

Since January, thousands of Gaza homeless have either lived with relatives or in UN-provided tents or in makeshift camps in the ruins of their homes. Some have built houses of mud bricks.

A political deal with Israel to ease its blockade of Gaza remains out of reach, blocked partly by the split in Palestinian ranks between Hamas Islamists who seized control of the enclave in fighting with the long dominant Fatah faction in 2007.

Donor countries pledged $4 billion for reconstruction at a meeting in January but no work can begin before Israel opens the border crossings it controls to building materials.

Anders-Frandzen said the UN still had $60 million of funds earmarked for reconstruction of war damage inflicted in Gaza in 2005 and still not cleared up.