JERUSALEM (CNN) -- A U.N. agency in Gaza will stop distributing aid to more than 750,000 Palestinians because Israel has halted border crossings into the Palestinian territory, the agency's director said Friday.
"We have run out of food. Our warehouses are empty," U.N. Relief and Works Agency Director John Ging said.
"These interruptions on the crossing points affect us immediately," Ging said earlier. "So the immediate impact is that on Saturday morning, we won't have any food to distribute."
U.N. Secretary-General Ban Ki-moon called on Israel "to allow urgently, the steady and sufficient supply of fuel and humanitarian assistance," according to a statement from the U.N. chief's spokesman.
Ban also called on Israel "to resume facilitating the activities" of UNRWA and other relief agencies by allowing U.N. officials and humanitarian workers unimpeded access to the region, the spokesman's statement said.
The Israeli military said it was forced to shut down the border because of rocket attacks on Israel and security threats.
The Israeli government denies that its actions have interfered with UNRWA's ability to provide assistance to some 80 percent of the territory's 1.5 million people.
Hostilities between Israel and Palestinian militants in Gaza continued for an 11th day Friday, bringing an effective end to a five-month cease-fire and creating what one U.N. official called a "desperate" humanitarian situation.
The Israeli military carried out airstrikes on a suspected militant rocket-launching site in northern Gaza early Friday, an Israel Defense Forces spokesman said.
The strikes targeted militants who had been firing rockets and mortars from Gaza into Israel. Five militants were wounded, according to Palestinian security forces.
Around a dozen rockets have been launched from Gaza in the last 24 hours, according to the IDF, with about 80 fired over the last 10 days. Shrapnel lightly wounded an Israeli woman in the southern border town of Sderot in one of the attacks.
Fighting between Israel and Palestinians flared up last week when Israeli troops entered Gaza to blow up a tunnel that the military suspected militants were going to use to kidnap Israeli soldiers.
That incursion set off a wave of fighting and rocket attacks in which 10 Palestinian gunmen have been killed and one Israeli soldier was wounded. It also prompted Israel to seal its borders Thursday, halting the already limited aid from reaching Gaza.
Gaza City went dark Thursday as a result of a power shortage, according to Palestinian officials. The Israeli government maintains that Gaza's Hamas leaders purposely cause power outages in the Palestinian territory in an attempt to garner international sympathy.
Ging said UNRWA usually stocks two months' supplies of rations, but the organization had been prevented from maintaining its strategic reserve during the five-month cessation of hostilities.
He characterized the policy as "a deliberate political decision" to keep the civilian population on the brink of a humanitarian crisis.
But Mark Regev, a spokesman for the Israeli prime minister's office, denied the allegation, saying, "We have been very cooperative with UNRWA."
Regev said, "We did not hear complaints that not enough aid" was being allowed to enter Gaza during the cease-fire.
He said recent Jewish and Muslim holidays probably had prevented the buildup of UNRWA's reserves.
Regev said the most recent round of fighting was instigated by Hamas, arguing that Israel's detonation of the tunnel was defensive and aimed at preventing a much larger escalation.
Regev said Israel was again prepared to abide by the truce if Hamas halted firing rockets.
CNN's Kevin Flower and Paul Colsey contributed to this report.