"If we don't get Karni open soon, a humanitarian crisis looms," Christopher Gunness, spokesman of the UN agency for Palestinian refugees (UNRWA), told AFP.

"Unless Karni is open, Palestinian exporters cannot export and Palestinian industry will start to die on its feet. Unemployment will start to rise and therefore the number of people reliant on emergency handouts is going to rise."

Israel sealed Gaza's borders when Hamas, whose charter calls for the destruction of the Jewish state, seized control of the territory on June 15 after routing Palestinian Authority security services in the territory.

The closure has raised alarm about the impact on Gaza, home to 1.5 million Palestinians who receive most of their food, and all their fuel and medical supplies via Israel, although Israel has allowed emergency deliveries in.

Although aid workers welcome shipments of emergency aid into Gaza through other crossings as staving off a threat of immediate humanitarian crisis, Gunness said wheat will run out in days and supplies of cement had already.

"There is no cement at all, so the UNDP (UN Development Programme) is considering closing down projects. We would have to halt housing assistance (rebuilding homes) for people living in abject poverty," the spokesman said.

"Our concern is that if we don't get Karni opening, there really will be a humanitarian crisis."

The World Food Programme, which intends to send 11 trucks of aid through the Sufa crossing on Wednesday, also warned of a serious situation in the Gaza Strip unless adequate supplies could get in and out.

"Unless a regular flow of goods into Gaza and out of Gaza can be maintained in order to meetings, then we're going to see stagnation of the economy and widespread aid dependency," spokeswoman Kirstie Campbell told AFP.

She said that UNRWA and WFP together provide aid to about 88 percent of the population in the Gaza Strip, but only 62 percent of its basic food needs.

An Israeli army spokesman said the Gaza crossings were closed -- despite specific deliveries of humanitarian aid and specific people allowed in and out -- owing to security reasons and no coordination on the Palestinian side.