Israel imposed the ban after discovering on Oct. 13 a 2.5-km (1-1/2-mile) tunnel under the Gaza border.

"We are in now in the fourth week (in which) we are not allowed to bring in construction material," Robert Turner, director of Gaza operations for the United Nations Relief and Works Agency (UNRWA), said on Tuesday.

"We do not know when we will be allowed to restart these projects," he said, calling for the ban to be lifted.

He told reporters that the only building project still under way, a bridge, was also running out of building materials. The others included 12 schools and a health center.

A spokesman for an Israeli agency that oversees shipments into Gaza gave no indication when the import ban might be lifted.

"For security reasons, building materials are not allowed into Gaza for the time being," he said.

UNWRA's Turner said the economic situation in the territory, where unemployment, according to UN figures, is at 30 percent, has worsened following Egypt's closure of tunnels under its border with Gaza.

Tunnels had provided a commercial lifeline for Gaza in the face of Israeli-led siege of Gaza.

Local economists say the tunnels' closure has forced about 20,000 Gazans out of jobs and caused consumer prices to spike.

"We see no positive indicators for Gaza. We do not see at any level or any sector where the situation is improving and that obviously is very worrying," said Turner, whose agency provides food aid to 830,000 of Gaza's 1.8 million population.

Israel imposed the all-out land, aerial, and naval blockade on Gaza in June 2007.

The siege has had a disastrous impact on the humanitarian and economic situation in the impoverished enclave, having turned the territory into the world’s largest open-air prison.

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