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."

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.

"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

Israeli military spokesperson Lt. Col Peter Lerner confirmed that hostages are influencing the plans of Israel's forces.

"Of course the presence of the hostages is at the top of our priority list," Lerner told ABC News. "It is obviously influencing our operational capabilities, operational plans."

Lerner said that while the military has been given the “green light” to go into Gaza, they have not officially been given the command to "go" from the government.

Asked if the window for an operation into Gaza will close, Lerner responded, "There is no choice for Israel."

Learner also said Israeli forces are actively trying to assassinate Hamas' leader in Gaza, Yahye Sinwar, but they haven't found him yet.

As the humanitarians conditions in Gaza become more dire by the day, Lerner said fuel will not be among the aid trickling into Gaza.

"Hamas has over a million liters of fuel in their stockpiles in Gaza -- they are actually not far away from Rafah. All they need to do is give some to the hospitals," he said.

-ABC News' Becky Perlow and Matt Gutman

The father of 23-year-old Hersh Goldberg-Polin, who was taken hostage by Hamas at the Supernova music festival, said he has gained some "strength" from seeing a video of his son on the day of the attack.

"No parent should ever be subjected to this sight," Jon Polin said on ABC News' "Good Morning America" on Tuesday.

Polin and Rachel Goldberg's son was wounded in the Oct. 7 attack. He had been hiding with a group in a bomb shelter and witnesses saw him being loaded into the back of a Hamas pickup truck, his parents told ABC News earlier this month.

Goldberg-Polin's parents said on Tuesday they have since seen a video in which their son leaves the bomb shelter.

"Knowing he spent an hour to an hour and a half being subjected to this massacre and he then gets up with an arm freshly blown off and walks on his own two feet, under his own strength, towards this truck and uses his weak hand, his only hand now, to pull himself onto the truck while bloodied, but looking sort of composed," Polin said. "It gives me a sense of, he's got a perseverance and fortitude that we hope carries him through this."

Israel's ambassador to the United Nations, Gilad Erdan, is calling on U.N. Secretary-General António Guterres to resign, saying he's "completely disconnected from the reality in our region."

At the United Nations Security Council meeting, Gutteres delivered some of most forceful rhetoric on the Israel-Hamas war to date, demanding an immediate humanitarian ceasefire “to ease epic suffering, make the delivery of aid easier and safer and facilitate the release of hostages.”

The Secretary-General’s speech, especially his assertion that “the attacks by Hamas did not happen in a vacuum," sparked immediate backlash from Erdan.

"The shocking speech by the UN Secretary-General at the Security Council meeting, while rockets are being fired at all of Israel, proved conclusively, beyond any doubt, that the Secretary-General is completely disconnected from the reality in our region and that he views the massacre committed by Nazi Hamas terrorists in a distorted and immoral manner," Erdan said. "His statement that, 'The attacks by Hamas did not happen in a vacuum,' expressed an understanding for terrorism and murder."

"It’s truly sad that the head of an organization that arose after the Holocaust holds such horrible views," Erdan said.

"There is no justification or point in talking to those who show compassion for the most terrible atrocities committed against the citizens of Israel and the Jewish people," Erdan said.

Guterres' spokesperson, Stephane Dujarric, told ABC News, "The Secretary said what he said. Spokesman is not responding further."

-ABC News' Shannon Crawford