The United Nations on Thursday warned its stocks had run so low that it would not be able to make its next delivery of food to 750,000 needy Gazans on Saturday.

"We've been working here from hand to mouth for quite a long time, so these interruptions on the crossing points affect us immediately," said John Ging, director of UN Relief and Works Agency operations in Gaza.

The Defense Ministry had said it would allow 30 truckloads of humanitarian supplies into Gaza on Thursday. But the crossings were kept shut because militants fired fire rockets and mortars into Israel earlier in the day, security officials said.

Later in the day, Defense Minister Ehud Barak ordered the army to keep the crossings shut after receiving an intelligence warning of a plan by Palestinian militants to attack the Kerem Shalom border terminal.

Gaza City was dark Thursday evening after a day of violence and retribution, raising the grim prospect of an end to a truce that has stopped most Israeli-Palestinian violence in and around the seaside territory for five months.

Gaza officials shut down their only power plant, cutting off electricity to much of the city of 300,000, after Israel canceled plans to ship in some diesel fuel for the plant as well as 30 trucks full of humanitarian supplies. The Israeli move came after Gaza militants fired at least eight rockets and some mortar shells at Israel on Thursday, according to the Israeli military.

Rocket fire has resumed over the past week after an armed clash in Gaza, and Israel has clamped a tight blockade on the impoverished seaside territory.

Though no one was hurt in the rocket attacks on Israel Thursday, Israel scrapped plans to allow small amounts of fuel and supplies into Gaza. Kamal Obeid, a Hamas official at of the power plant, said fuel was running out and the facility would be shut down completely later Thursday.

Israelis counter that the plant provides less than a quarter of Gaza's electricity, and most of the rest flows in unimpeded on power lines from Israel.

Speaking in Brussels, Belgium, UNRWA head Karen AbuZayd said it was unusual for Israel not to let basic food and medicines in.

"This has alarmed us more than usual because it's never been quite so long and so bad, and there has never been so much negative response on what we need," she said.

"We have hundreds of containers waiting in Ashdod port, holding such simple things such as the wool and the yarn for vocational training centers or centers for the visually impaired to make some money," she said. "We were told these are not humanitarian supplies."

Israel has not allow the UN and other agencies to bring supplies into the Gaza Strip since Nov. 4, when its troops raided the territory to destroy what the army described as a tunnel built by militants.

Six Hamas gunmen were killed in the operations. Gaza militants responded to the incursion with rocket salvoes at southern Israel.

The ceasefire, which began in June, calls on both sides to stop cross-border violence and on Israel to ease the Gaza blockade it tightened after Hamas Islamists seized the territory more than a year ago.

Israel: Truce only to resume when Hamas keeps its commitments

Senior Defense Ministry official Amos Gilad on Thursday said Israel's truce with Hamas in Gaza, ruptured by a flare-up in cross-border fighting, will resume only when the Palestinian militant group keeps its commitments.

"The lull will return only when we are convinced that Hamas has gone back to its commitment to keep the lull, because it decides when [militants] shoot and when they don't," said Gilad, speaking on Israel Radio.

Gilad defended Israel's closing of the border crossings against claims that it would prevent Gazans from meeting their most basic needs.

"Israel is working to prevent the occurrence of a humanitarian crisis in Gaza, but will not endanger its soldiers by continuing to supply produce - because Hamas even attacks the crossings' terminals," he told Israel Radio.

"The murderous attacks must end. We are sensitive to the humanitarian situation but there is a serious concern that murderous and terrorist acts will take place."

Israel also held up shipments of European Union-funded fuel to the territory's sole power plant. Palestinian officials said the facility would be shut down later in the day.

Israel blocks entry of 20 EU diplomats into Gaza

Also Thursday, Israel prevented 20 European Union consul generals from entering Gaza on Thursday after a recent upsurge in clashes between the Israel Defense Forces and Gaza militants.

The consuls had planned to meet with businesspeople and human rights activists in the Hamas-ruled coastal territory in order to learn about the humanitarian situation.

Early in the day, Palestinian mortar and rocket gunners bombarding southern Israel after the IDF killed four Hamas gunmen in the Strip on Wednesday.

A senior official at the French Consulate, which had arranged the trip, said he could not remember another time when Israel had prevented diplomats from entering Gaza over reasons that were not security-related.

Israel Defense Forces spokesman Peter Lerner told Haaretz that Israel had informed the diplomats on Wednesday of its intention to prevent their entry, and yet they still attempted to cross into Gaza.

"The reason their entry was prevented was because it was not humanitarian. The policy today is only to allow the most essential entry... I hope [Gaza militants] will stop shooting missiles and then we can return to the previous situation," he said.