The UN agency that looks after Palestinian refugees said it could be forced to halt operations in the Gaza Strip unless fuel is delivered to the territory.
"If we do not get fuel urgently, we will be forced to halt our operations in the Gaza Strip as of [Wednesday] night," the UNRWA said on X, formerly Twitter.
Aid groups in Gaza have sounded the alarm about the lack of fuel for vital services such as hospitals that rely on generators.
"If we do not get fuel that will allow us to continue delivering humanitarian assistance, we will not be able to continue to assist people in need beyond Wednesday night," the UNRWA's director of communications Juliette Touma told the DPA news agency.
Touma said that 600,000 internally displaced people in more than 150 shelters depend on assistance from UNRWA.
The United Nations says 20 trucks due to deliver aid to the besieged Gaza Strip through the Rafah crossing from Egypt did not enter the enclave as planned on Tuesday.
Since Saturday, 54 trucks have crossed into Gaza carrying food, medicine and water.
World leaders seek pause to allow aid into Gaza
World powers called for a pause in fighting between Israel's army and the Islamist militant group Hamas to allow more aid to enter the Gaza Strip.
A pause is generally considered less formal and shorter than a cease-fire.
"While we remain opposed to a cease-fire, we think humanitarian pauses linked to the delivery of aid that still allow Israel to conduct military operations to defend itself are worth consideration," a senior US official told Reuters.
At the UN Security Council on Tuesday, the US and Russia presented rival plans to get humanitarian aid to Palestinians.
"The whole world is expecting from the Security Council a call for a swift and unconditional cease-fire," Russia's UN Ambassador Vassily Nebenzia said
Arab states have also called for a humanitarian cease-fire.
"We followed with regret the inability of this council twice to adopt a resolution or even to call for a cease-fire to end this war," Egypt's Foreign Minister Sameh Shoukry told the council.
Some world leaders like Canadian Prime Minister Justin Trudeau also expressed interest in a pause in fighting for humanitarian purposes.
"There are a lot of conversations going on now about the need for humanitarian pauses and I think that's something Canada supports," he told reporters in Ottawa.