GAZA CITY, Gaza Strip - A United Nations flour warehouse in Gaza that was full a week ago now stands empty -- the guttural sounds of trucks replaced by the chirping of pigeons in the rafters.

Another warehouse holds just a few crates of lunch meat and space usually filled with oil and powdered milk is taken up by air conditioners for medical centers yet to be built.

A week after Israel closed Gaza's borders in response to rocket fire by Palestinian militants, the UN aid agency warned its stocks had run so low it would be forced to halt food distribution to the needy.

Besides providing food aid to 750,000 Gazans, the UN Relief Works Agency runs schools, medical clinics and other programs. The Israeli closures affect all the agency's activities, said UNRWA spokesman Christopher Gunness.

Late on Wednesday, Israel's Defense Ministry said it would allow 30 truckloads of humanitarian supplies into Gaza. Gunness said the shipments would ease some shortages but that Israeli restrictions have been forcing the UN to operate in crisis mode.

"It is not good enough for the UN to be pushed to the brink every time," he said.

The crisis is only the latest since Islamic militants from Hamas overran the Gaza Strip last year. Hamas and Israel are bitter enemies. Hamas does not recognize a place for a Jewish state in an Islamic Middle East and has sent dozens of suicide bombers into Israel, which in turn labels Hamas a terror group.

To pressure Hamas, Israel imposed a blockade, allowing only minimal humanitarian supplies and an occasional trickle of commercial goods into the territory. All but one of Gaza's crossings are into Israel. The exception is Rafah, which leads to Egypt but Egypt is also enforcing a blockade.

Among the items UNRWA has not been able to get into Gaza are fire extinguishers for its facilities, toner for the photocopiers in its schools and clinics and materials for a blind children's center, Gunness said.

"These children are effectively being punished as a group, and it's hard to see why they should be punished for a small group of people firing rockets," Gunness said.

More than half of Gaza residents are refugees and their descendants from the 1948-49 war over Israel's creation and many still live in squalid shantytowns.

The Israeli blockade has plunged the crowded territory even further into poverty, while keeping construction materials out and Gazans locked in. About 80 percent of Gaza's 1.4 million residents depend on food aid, according to UN figures.

Israeli military spokesman Peter Lerner said the crossing was closed in response to rocket fire into Israel from Gaza. No decision had yet been made about when to reopen the crossings but the government was considering the UN's position, Lerner said.

However, "If Hamas continues firing rockets into Israel, it impedes our ability to open the crossings," he said.

The UN distributes food aid to Gazans in cycles. Families are categorized by size and each group has a window every two or three months when it can pick up its food.

When the UN is forced to stop distribution, the tens of thousands of people eligible to get their food during that period will get nothing, said UN officials. This will delay the cycle, meaning that all food recipients will have to wait longer for their next installment.

On Wednesday afternoon, workers in the center in the Shati refugee camp next to Gaza City prepared bags of rice, flour, oil and powered milk for the 250 people they expected to come the next day.

"If there's nothing there it will be a disaster for people here," said Adil Adwalla, 35, a construction worker who lives nearby. He can't find work because the blockade has made building materials scare, he said. That makes the food aid even more important to his wife and six children, he said.

The center had no lunch meat for more than a week and had only enough oil for two days. It would run out of milk the next day, the center's supervisor said. He did not give his name because he is not authorized to speak to reporters.

"With meat, people accept it, but when it's milk, it's much harder. We don't know how to make people understand it's not us," he said. "The crossings are closed but they think it's our fault."