Israel also said on Friday it will maintain its closure of the Gaza Strip despite international concern over a deterioration of the humanitarian situation in the aid-dependent Palestinian territory.
Since the recent surge in violence, only basic supplies and limited quantities of fuel have been allowed into the impoverished Gaza Strip.
The UN Relief and Works Agency (UNRWA), which distributes rations to half of Gaza's 1.5 million population, said it had food stocks for "days, [but] not weeks."
"In the coming days we expect some food items to run out," Chris Gunness, a UNRWA spokesman, told the AFP news agency.
The Israeli blockade has already forced the UNRWA to suspend its food deliveries once last week.
The organisation suspended the distribution of rations last Friday before resuming on Tuesday after some supplies were allowed in the previous day.
An Israeli defence ministry spokesman said that rocket fire from Gaza towards Israel had prompted the decision to reseal the crossings on November 4.
"This decision was taken because of the continuation of Palestinian rocket attacks against southern Israel," Peter Lerner, a defence ministry spokesman, said.
His comments came after a rocket fired from Gaza hit the port city of Ashkelon early on Friday without causing casualties or damage, according to Israeli army radio.
The truce between Israel and Hamas, which is not involved within the US-sponsored peace talks, is due to expire next month.
Tony Blair, the envoy of the Middle East diplomatic Quartet, made up of the European Union, Russia, the United Nations and the United States, has urged Israel to reopen the border crossings immediately.
"The immediate reopening of Gaza for the regular entry of essential humanitarian and commercial goods, including fuel, food and medicines is vital," the former British prime minister said.