Gazans seeking food aid walked away empty-handed from locked United Nations distribution centers Saturday after a strict Israeli border closure depleted U.N. food reserves.

Israel sealed Gaza's borders nearly two weeks ago as part of a new round of fighting with Gaza's Hamas rulers. Hamas rocket fire on Israeli border towns and Israeli air strikes on Gaza militants have eroded a truce that had largely held for five months.

U.N. Secretary-General Ban Ki-moon called on Israel to open the crossings to humanitarian aid and condemned the rocket fire on Israel. Measures that increase the suffering of Gaza's civilians "are unacceptable and should cease immediately," he said in a statement.

Two Palestinians were killed in disputed circumstances in northern Gaza.

Palestinian Health Ministry official Dr. Moawiya Hassanain, citing reports from local medics, said the two were killed by an Israeli airstrike. However, the military said Israeli forces were not involved and in the past, militants have sometimes been killed by Gaza rockets that fell short or exploded early.

In the Shati refugee camp near Gaza City, hundreds of people walked away empty-handed from a U.N. food distribution center Saturday. A note taped to the center's blue gate said handouts were put off until Dec. 13 "because of a lack of food to distribute."

In all, the U.N. Relief and Works Agency distributes food to some 750,000 Gazans, or nearly half the territory's population. The needy get a new parcel of rice, flour, sugar and oil every three months. On Saturday, some 20,000 Gazans were to pick up food supplies, U.N. aid officials said.

Itaf Yazji arrived at the Shati distribution center Saturday, only to find it locked.

"What shall we eat now?" said the 54-year-old mother of five, who also cares for a disabled relative. Yazji said she had been waiting anxiously to pick up food because her family ran out of rice and flour.

The U.N. World Food Program, which feeds another 130,000 Gazans, says it has enough food to distribute for the next four weeks.

Most of Gaza's 1.4 million residents live in poverty that has deepened since Israel and Egypt imposed a blockade on the territory after militant group Hamas seized power in July last year.

The cease-fire began to deteriorate last week after an Israeli military raid on what the army said was a tunnel that militants planned to use for a cross-border raid. At least 11 militants have been killed since, not including the latest casualties. And some 140 rockets and mortars have been fired from Gaza at Israel. They include four Grad-type Katyushas that landed in Ashkelon on Friday, some 11 miles from Gaza.

Under the blockade, Israel is meant to allow in humanitarian aid, rationed fuel and some commercial goods. Israel says the closure was imposed in response to continued violence.

In the West Bank town of Ramallah, a Palestinian negotiator Saeb Erekat said President Mahmoud Abbas will meet with the outgoing Israeli prime minister on Monday and raise his concern about the Gaza fighting.

The two leaders have met regularly for the past year as part of U.S.-backed peace talks. However, all sides have acknowledged that they will miss the end-of-year target for a peace deal. Olmert has three months left in office.