There is a place of strange quiet in the cramped and crowded Gaza Strip. It looks, from the roof of a nearby United Nations school, like a film set, or perhaps an army's urban warfare training ground. Ranged across the sandy earth of Khan Younis is a large housing estate: 151 apartments, with space for a further 450. Most are three-quarters complete. All are uninhabited.

The project is one of 26 schemes, ranging from houses to schools to medical clinics, that have been years in the making. They all made good progress until June 2007. At that point, the Islamist Hamas movement - which has fired hundreds of rockets at southern Israeli towns - took control of the Gaza Strip after months of violent struggle with its more secular rival, Fatah. In response, Israel and Egypt tightened their blockade of the Gaza Strip, allowing in little more than basic food and medicine.

For the past 10 months, the UN has been holding intensive, high-level negotiations with Israel, seeking permission to bring in materials such as doors, windows, pipes and tiles to complete these 26 projects. But UN officials say they have made no headway. Their expressions of dismay are growing stronger.

'Huge price'

Fouad Faqawi, a Gazan who works for the United Nations relief agency Unrwa, strides up the roug concrete staircase of one of the Khan Younis housing blocks. "Nobody can live here," he says, pausing to look inside the shell of a family home for six. "No way - how can people live without plumbing or sewage, or windows or doors?" Children in shack in area where Maryam Ataya is living
Nearly 100 people, including many children, live in this collection of shacks Unrwa's head in Gaza, John Ging, surveys the housing estate on a grey and windy winter's day. His voice crackles with incomprehension and frustration, as he talks of the people waiting for the new homes to be finished.
"These are civilians, who are of course the victims of this conflict. And yet they're paying this massive price, in terms of human misery. And the frustration and despair are creating a lot more extremism."

Barbed wire

Were the estate finished, it might benefit Maryam Ataya. Along with almost 100 relatives, she lives in a squalid little enclosure a short drive away. Their home was destroyed during the Israeli offensive last year. Now, she and her four children live in a collection of shacks, huts and lean-tos.

Children careen bare-footed over the mounds of rubble, barbed wire, and rusted metal within the rickety perimeter walls. "It's a disaster," says Maryam flatly. She stokes the family pot using scraps of wood. "We have no electricity, no running water."

The Israeli government's general position on the blockade is that it will remain in place - in the words of a senior official - "as long as Hamas remains committed to destroying Israel and killing Israelis". But what of these specific UN projects? In a statement, the defence ministry told the BBC: "Recently, the UN began to submit detailed equipment lists (for the 26 projects). Once the administrative work is completed, it will be agreed with the UN… which projects will be realised, and the timetables for their execution."

Strong language

The 26 schemes have become known as the "Serry projects", after Robert Serry, the UN special co-ordinator for the Middle East peace process. The mild-mannered, quietly spoken Dutch diplomat speaks in the UN's Jerusalem headquarters with a clear tinge of exasperation.

"Let make this very clear," he says. "I am disappointed and also frustrated, that after months of discussions… Israel is not yet willing to discuss any of the social housing projects which the (UN) secretary general has been asking Israel now to move on. Frankly, we're getting impatient."

For the UN envoy, this is strong language.

Israeli officials have long warned of the danger that Hamas could divert building materials for military ends, such as bunkers and reinforcements. But the UN stresses that every single tile, pipe or bag of cement is tracked from the border crossing to its final use. "I fail to see how these kinds of projects, which would help the people of Gaza - not Hamas - would impact on Israel's security," Mr Serry adds.

And at Abdelsalaam Al-Shobaki's small concrete factory, in the north of Gaza, there is proof that the blockade has failed to seal the strip. There is a large pile of empty packets of cement from Turkey and Egypt, which have been smuggled through the tunnels connecting Gaza to Egypt.

As he mixes the grey slurry, Mr Shobaki says the much-needed building material is punishingly expensive. But the cement is clearly there, if you have money or power - and Hamas has both. There is no sign that the struggle between Hamas and Israel will be eased any time soon. As long as that is the case, there seems little prospect of Gaza's borders opening. And that will leave Gazans mired in lives of privation and shortages.