A United Nations aid agency on Tuesday said it would have to halt food distribution to 750,000 needy Gazans by Friday if Israel kept the territory sealed.
Agency spokesman Chris Gunness said food stockpiles are running low. Israel has not said when it will re-open the crossings.
It had closed them and blocked shipments of European Union-funded fuel for a week in response to a surge in rocket attacks by Palestinian militants, who said they were responding to an Israel Defense Forces raid that killed six gunmen on Nov. 4. The army said the raid was launched in order to foil the planned kidnapping of an IDF soldier by Gaza militants.
Earlier Tuesday, Israel renewed fuel deliveries to the Gaza Strip, ending a week-long suspension of supplies that led to blackouts in the Hamas-controlled enclave where militants had launched cross-border rocket attacks.
About half of Gaza's 1.5 million residents lost power on Monday when their sole power plant shut down, due to what Palestinian officials said was a lack of fuel.
The violence disrupted a ceasefire that began in June.
Palestinian workers said the first delivery was received at the Nahal Oz fuel depot and was on its way to the power plant.
Israel is the sole provider of industrial fuel for Gaza's only power plant. The officials said Barak agreed to a request from peace envoy Tony Blair, and Israel will allow minimal amounts of fuel for the power plant to enter Gaza starting Tuesday. Otherwise, the border crossings will remain closed, they said.
The decision to resume the deliveries came despite a rocket attack on Israel Monday afternoon. The military said Palestinian militants fired three rockets at Israel, but two fell short and exploded inside Gaza.
The Israeli closure has also kept journalists from entering the coastal territory for five days, drawing a protest from Tel Aviv-based Foreign Press Association. The FPA, which represents foreign reporters in Israel and the Palestinian territories, called the Israeli move a serious violation of press freedom.
Israeli military spokesman Peter Lerner said crossings would open only for humanitarian reasons, and that journalists would be allowed to leave but not enter.
Gaza energy official Kanan Obeid said the power plant was shut down after sundown. Much of Gaza City was without electricity after nightfall, as some residents started up their own generators.
In a statement, Israel's Foreign Ministry said 70 percent of Gaza's electricity comes over lines directly from Israel, and that continues to flow. Another 17 percent is supplied by Egypt, the statement said, adding that Israel remains committed to the truce.
Gaza residents purchase gas and diesel fuel smuggled in from Egypt, which is expected to alleviate the blackouts as Palestinians switch to private generators. A Palestinian health official said he did not expect any immediate problems because the Health Ministry had backup fuel, but a prolonged cutoff would cause hardship.
Hamas officials warned the Israeli move could further undermine the shaky truce.
"We warn that we have many options, and we will not stand idly by and watch our people die a slow death," said Taher Nunu, a spokesman for the Hamas government in Gaza.
Deputy Defense Minister Matan Vilnai said Hamas is using Gaza's electricity to attack Israel.
"It's a pity that we are falling for this propaganda. I can confirm one thing, if there is only one kilowatt in all of Gaza it will be in one place only - a workshop that manufactures rockets," he told Army Radio. "They are using us. They know us. They know our sensitivity to humanitarian issues and they are playing with it."
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