The UN says it has halted work on all but one of its 20 building projects in the Gaza Strip as a result of an Israeli ban on importing building materials into the Palestinian territory.
Israel imposed the ban after discovering on October 13 a 2.5km tunnel which it said fighters planned to use for attacks inside its territory.
Hamas, which has run the Gaza Strip since 2007, claimed it dug the tunnel.
"We are in now in the fourth week [in which] we are not allowed to bring in construction material," said Robert Turner, director of Gaza operations for the United Nations Relief and Works Agency (UNRWA).
"We do not know when we will be allowed to restart these projects."
UNRWA provides food aid to 830,000 of Gaza's 1.8 million population.
Turner called for the ban to be lifted, adding 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 centre.
A spokesman for the Israeli government 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 smuggling tunnels under its border with Gaza.
Egypt's military-run government, which has been battling fighters in the Sinai Peninsula near Gaza, regards Hamas as a security threat.
Hamas has denied Egyptian accusations that it has been involved in Egyptian unrest.
Tunnels had provided an commercial lifeline for Gaza in the face of Israeli-led economic sanctions.
They were also used by armed groups to smuggle in weapons and funds.