UN has no more food to distribute in the Gaza Strip, the head of relief efforts in the area has warned, BBC reported on Thursday.
John Ging said handouts for 750,000 Gazans would have to be suspended until Saturday at the earliest, and called Gaza's economic situation "a disaster".
Israel earlier denied entry to a convoy carrying humanitarian supplies. It has prevented the transfer of all goods into Gaza for nearly a week, blaming continuing rocket attacks by Palestinian fighters.
The United Nations Relief and Works Agency (Unrwa) distributes emergency aid to about half of Gaza's 1.5m population. "We have run out [of food aid] this evening," said Mr Ging, Unrwa's senior official in Gaza. "Unless the crossing points open... we won't be able to get that food into Gaza," he told a news agency.
Also on Thursday, Israel refused permission for a group of senior European diplomats to visit the coastal enclave. It has also prevented journalists, including those from the BBC, from entering the territory.
Limited supplies of fuel were sent over on Tuesday after Gaza's only power plant ran out of diesel. The fighters say the mortar and rocket fire is their response to what they say is Israeli aggression against Gaza.
Israeli government spokesman Mark Regev insisted any improvement would be dependent on the Hamas Movement, which runs the Gaza Strip.
"There's been a combat situation and it's very difficult to have unhindered functioning of the border crossings in a situation where shooting is going on," he said.
On Wednesday, Israeli troops killed four Palestinian fighters from the Hamas Movement.
Witnesses said fighting broke out on the Gaza border after Israeli armoured vehicles crossed into the territory near Khan Younis. The army said its soldiers were trying to stop the freedom fighters plant a bomb near the security fence surrounding the strip.
The Gaza power plant provides most of the electricity used in Gaza City; Israel supplies most of the rest of the territory's energy needs, but the system is liable to become overloaded and blackouts are common.