Israel blocked humanitarian supplies from entering the Hamas-controlled Gaza Strip and the United Nations said it would be forced to suspend food distribution on Thursday.
In addition to preventing aid agencies from delivering food to 1.5 million Gaza residents, Israel held up shipments of European Union-funded fuel to the territory's sole power plant. Palestinian officials said the plant would be shut down later in the day.
"They have told us the crossings are closed today. At the end of today we will suspend our food distribution," said UN Relief and Works Agency spokesman Chris Gunness.
"Our warehouses are effectively empty," he told AFP.
The International Committee for the Red Cross (ICRC) said a truck it sent to the Kerem Shalom crossing was turned back.
Israel usually allows some humanitarian supplies into Gaza, but even this has stopped over the past week, leading to harsh criticism from aid agencies and human rights organizations.
"Pushing people to the brink of desperation every few months and forcing UNRWA into yet another cycle of crisis management is not in the interest of anyone who believes in peace, moderation and stability," said Gunness.
“The imposition of the blockade on Gaza by Israel with the cooperation of Egypt is a clear violation of international law and constitutes collective punishment,” Joe Stork, deputy director of Human Right Watch’s Middle East division, told AlArabiya.net.
“The Israelis have been careful calibrating what they let in so there is no death by starvation,” though he added that Gazans do not have a healthy diet or get enough to eat.
ICRC mission chief Katharina Ritz said that "every day the situation is getting more and more precarious for Gazans," adding that there was a desperate need for medical supplies.
Israel had initially said it would allow the delivery of fuel and some 30 truckloads of food and other humanitarian supplies into the enclave, where a flare-up in cross-border fighting threatens a 5-month-old truce.
Several rockets were fired at southern Israel earlier on Thursday, causing no casualties, the Israeli army said, a day after soldiers killed four Hamas forces during a raid into the Gaza Strip.
UNRWA usually distributes emergency food rations to about 750,000 people in the impoverished, overcrowded sliver of land whose economy has been crippled by a tight blockade Israel says is aimed at forcing militants to stop firing rockets and mortar rounds at the Jewish state.
The Israeli military confirmed the closure of Gaza continued on Thursday.
"The crossings will remain closed today for security reasons," defense ministry spokesman Peter Lerner said.
Reopening the crossings "has been delayed because of the mortar shelling that impedes the proper functioning of the crossing points," he said.
Gunness said Israeli officials were non-committal about allowing in supplies on Friday. The crossings are generally closed mid-day Friday through Sunday.