At Gaza's Al-Shifa Hospital, 43 out of the 63 intensive care patients have died as oxygen in the intensive care unit runs out, according to Dr. Ahmed Mokhallalati, head of the hospital's plastic surgery department.

Mokhallalati told ABC News the mission of burying bodies is ongoing as more people die inside and outside the hospital.

Mokhallalati said he could still hear the Israeli tanks at the hospital gates Wednesday night.

Twenty-two of Gaza's 36 hospitals are now "non-functional," the World Health Organization said Wednesday.

The "14 hospitals remaining open have barely enough supplies to sustain critical and lifesaving surgeries and provide inpatient care," the WHO warned.

The organization in a statement reiterated its calls for a cease-fire, protection of civilians and "respect for international humanitarian law."

The Israel Defense Forces said its operation at Gaza's Al-Shifa Hospital complex is ongoing.

The IDF said its forces "engaged with" and killed "a number of terrorists" when entering the hospital complex.

Following searches in the hospital, the IDF said its troops "located a room with technological assets, along with military and combat equipment used by the Hamas terrorist organization."

Palestinian journalist Khadr al Zanoon, who is at the hospital, told ABC News no fighting has taken place inside, but he can hear tanks outside.

He said Hamas fighters are not in the hospital but are in the area around it and are fighting with Israeli forces.

He said Israeli forces have detained some Palestinians who were inside the hospital.

The raid on Al-Shifa Hospital began early Wednesday around 3 a.m. local time, after Israeli forces had moved closer to the medical complex for several days.

Thousands of civilians, along with hundreds of patients -- most of whom are seriously ill -- have been sheltering at Al-Shifa, according to hospital staff and Gaza health officials.

The IDF alleges that Hamas has placed its command centers under Al-Shifa and other hospitals in Gaza and is deliberately sheltering behind Palestinian civilians -- claims that the militant group denies.

National Security Council spokesperson John Kirby on Wednesday denied that the U.S. gave any "OK" for the Israeli operation at Gaza's Al-Shifa Hospital.

"These are Israeli military operations that they plan and they execute on, you know, in accordance with their own established procedures, that the United States is not, was not, involved in," Kirby said.

He also denied that the U.S. confirming intelligence that Hamas uses the hospital as a control center had anything to do with the timing of the Israeli military operation at the hospital, which began only hours after Kirby's announcement.

Kirby also said Israel's hospital operation was "not a focus" of President Joe Biden's Tuesday night conversation with Israeli Prime Minister Benjamin Netanyahu and would not say if the U.S. got a heads up about the operation.

"Again, we don't expect the Israelis to advise us or inform us when they are going to conduct operations," Kirby said. "We talked to them routinely every day, and certainly we talked to them about our continued concerns over civilian casualties and sharing our perspectives on the best way to minimize, but these are their operations."

-ABC News' Sarah Kolinovsky

Patients and doctors evacuated the Al-Shifa hospital on Saturday, after the Hamas-run Gaza Health Ministry claimed Israel forced them to leave. The IDF released a statement denying it ordered the evacuation, but in a Friday briefing another spokesperson said the IDF was urging anyone left in Al-Shifa hospital to leave and that it hoped it would take place in the "next few hours."

Doctors described their exodus from the hospital to ABC News, with Dr. Ahmed Mokhallalati -- who is still in the hospital -- saying "all critical ICU patients have died. The situation is terrifying."

"Today early in morning people were forcefully evacuated from the hospital, with absolutely no plan of evacuation of the patients and the medical staff. Most of the civilians together with most of the staff left the hospital," Mokhallalati said.

Mokhallalati said there are around 300 patients, who cannot move, and less than 50 medical staff still in the hospital.

"There are still 33 premature babies in the hospital -- one baby died yesterday, and two babies were taken by their parents to evacuate with them. There is only one neonatologist and one nurse with them," he added. The Israelis have provided only three transport incubators for 33 babies. So, if they tell us 'you have to evacuate now,' I have no idea how to evacuate them."

Dr. Adnan Al-Barash, head of the orthopedic department at Al-Shifa Hospital, told ABC News that the "Israeli army forced us to leave the hospital at gunpoint."

"The path for us to walk was set out among the tanks, we had elderly, wounded… The scene was very tragic and sad," Al-Barash said.

"We went out between the tanks, and we could not get the wounded out on the broken roads and we could not move wheelchairs for the wounded," he added.

-ABC News' Dragana Jovanovic and Zoe Magee