Without fuel, the agency said it'll "be forced to halt our operations."
Thousands of people have died and thousands more have been injured since the militant group Hamas launched an unprecedented surprise attack on Israel on Oct. 7 and Israel retaliated with a bombing campaign and total siege of the neighboring Gaza Strip, leaving the region on the verge of all-out war.
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The latest outbreak of war between Israel and Hamas, the Palestinian militant group that governs the Gaza Strip, has passed the four-month mark.
In the Gaza Strip, at least 30,228 people have been killed and 71,377 others have been wounded by Israeli forces since Oct. 7, according to Gaza's Hamas-controlled Ministry of Health.
In Israel, at least 1,200 people have been killed and 6,900 others have been injured by Hamas and other Palestinian militants since Oct. 7, according to the Israel Defense Forces.
There has also been a surge in violence in the Israeli-occupied West Bank. Israeli forces have killed at least 395 people in the territory since Oct. 7, according to the Palestinian Ministry of Health.
The ongoing war began after Hamas-led militants launched an unprecedented incursion into southern Israel from neighboring Gaza via land, sea and air. Scores of people were killed while more than 200 others were taken hostage, according to Israeli authorities. The Israeli military subsequently launched retaliatory airstrikes followed by a ground invasion of Gaza, a 140-square-mile territory where more than 2 million Palestinians have lived under a blockade imposed by Israel and supported by Egypt since Hamas came to power in 2007. Gaza, unlike Israel, has no air raid sirens or bomb shelters.
The Israeli police said they've identified at least 784 people killed by Hamas.
Police said some bodies were in such bad condition that they have not yet been identified.
At least 1,400 people have died and 4,629 others have been injured in Israel since Hamas' Oct. 7 attack, according to Israeli authorities.
The Biden administration has sent Lt. Gen. James Glynn, a three-star Marine general who is currently serving as the head of Marine personnel, to Israel to advise the country on its military operations, according to a U.S. official.
The news was first reported by Axios on Monday.
Glynn is "not directing operations" but rather is "purely there to provide military advice and pose hard questions to help [the Israel Defense Forces] think through various scenarios," the U.S. official told ABC News.
The official said Glynn was in Israel "temporarily" and was not expected to still be there when a ground operation starts.
-ABC News' Ben Gittleson
Fuel is now the "most vital commodity" in Gaza, according to the World Health Organization.
The limited aid trucks trickling into Gaza have not included any fuel, the organization said. Before Oct. 7, hundreds of trucks entered Gaza every day, including about 45 trucks bringing fuel, said Tamara Alrifai, spokesperson for the U.N. agency for Palestine refugees.
Without fuel, "trucks can’t move and generators can’t produce electricity for hospitals, bakeries and water desalination plants,” said Alrifai.
Alrifai said the United Nations Relief and Works Agency would be responsible for delivering the fuel to hospitals and water desalination plants and keeping it out of Hamas' hands.
The WHO said one in three hospitals in Gaza and two in three clinics are not functioning, with the health system overwhelmed by more than 16,000 injured people.
Dr. Rick Brennan, WHO emergencies director for the Eastern Mediterranean Region, said he's begging "all those in a situation to make a decision or influence decision makers, to give us the humanitarian space to address this human catastrophe."
Young journalist in Gaza describes life without power
In just two weeks, the space below Jerusalem's Hadassah Hospital has been converted into an underground hospital, bracing for an influx of war casualties.
Rows of hospital beds and medical equipment have been set up in what was meant to be used as a parking garage.
"We have up to 130 beds here, including intensive care beds," Dr. Tamar Elram, director of the Hadassha Mount Scopus Hospital, told ABC News. "Everything that we do here is in total cooperation and agreement with the army and with police and all the other security forces."
The hospital has also been treating civilian victims, like Michal Alon, who was shot in the hand and chest by Hamas militants on Oct. 7 and is now embarking on the long road to recovery, both physically and emotionally.
Israeli hospital prepares for war casualties
"We've already got soldiers and civilians who are turning to our ERs, two and a half weeks after the terror attack, starting to suffer from acute post-traumatic syndrome," Elram said.
Elram says one of the biggest challenges they've faced in preparing for what's to come includes manpower. Some staffers are leaving the hospital to go serve in the Israeli military.
-ABC News' Guy Davies and Ines De La Cuetara
