UN Secretary-General Antonio Guterres has called for Israel to re-open Gaza border crossings it closed after launching the war against Iran along with the United States.
Among the crossings shut down on Saturday as a security measure is the Rafah conduit -- the only gateway for Gazans to the outside world that does not pass through Israel.
It had reopened for the movement of people on 2 February, nearly two years after Israeli forces seized control of it during the war with Hamas.
"Israeli authorities have closed all crossings, including Rafah, and have suspended humanitarian movements in and near areas where Israeli troops remain deployed in Gaza," said spokesman Stephane Dujarric.
"It is imperative that all crossings be reopened... as soon as possible. In recent days, our partners have been forced to ration fuel, prioritize life-saving operations, albeit in reduced capacity as our local stocks are going down."
Mr Dujarric said that there were some stockpiles already in Gaza but that "when the doors are shut, we obviously stretch whatever we have to make it last longer."
Gaza to run out of fuel in 'a couple of days'
Gaza is rapidly running out of its limited fuel supply and stocks of food staples may become tight, officials warn, after Israel blocked the entry of fuel and goods into the war-shattered territory, citing fighting with Iran.
Israel's military closed all Gaza border crossings on Saturday after announcing its air strikes on Iran, conducted with the US.
Israeli authorities say the crossings cannot be operated safely during war and have not said how long they would be shut.
Gaza is wholly dependent on fuel brought in by trucks from Israel and Egypt and a lack of fresh supplies would put hospital operations at risk and threaten water and sanitation services, local officials say.
Most Palestinians in Gaza are internally displaced after Israel's two-year war with Hamas militants.
"I expect we have maybe a couple of days' running time," said United Nations official Karuna Herrmann, who directs fuel distribution in Gaza.
Amjad Al-Shawa, a Palestinian aid leader in Gaza, who works with the UN and various charities, estimated fuel supplies could last three or four days, while stocks of vegetables, flour, and other essentials could also soon run out if the crossings remain shut.
Reuters was unable to independently verify those estimates.
Israel's COGAT military agency, which controls access to Gaza, claimed that enough food had been delivered to the territory since the start of an October truce to provide for the population.
"(The) existing stock is expected to suffice for an extended period," COGAT said, without elaborating. It declined to comment on potential fuel shortages.
That truce was part of broader US-backed plan to end the war that involves reopening the Rafah border crossing with Egypt, increasing the flow of aid into the enclave, and rebuilding it.
Hamada Abu Laila, a displaced Palestinian in Gaza, said the closures were stoking fear of a return of famine, which gripped parts of the enclave last year after Israel blocked aid deliveries for 11 weeks.
"Why is it our fault, in Gaza, with regional wars between Israel, Iran, and America? It is not our fault," Abu Laila said.
