When a reporter on Friday asked President Joe Biden what worries him about a potential Israeli ground invasion into Gaza, Biden responded, "Death."

UN Secretary-General António Guterres warned earlier Friday that Israel’s call for more than 1 million people to evacuate northern Gaza "is extremely dangerous, and in some cases, simply not possible."

He warned that "hospitals in the south of Gaza are already at capacity and will not be able to accept thousands of new patients from the north."

"The health system is on the brink of collapse; morgues are overflowing," Guterres said at the UN headquarters in New York. "The entire territory faces a water crisis, its infrastructure has been damaged, and there is no electricity to power pumps and desalinization plants."

Israeli forces have postponed the demand to evacuate Gaza's Al Awda Hospital until 6 a.m. local time.

-ABC News' Ben Gittleson and Mary Bruce

While the U.S. and Israel have been coordinating efforts to lessen civilian casualties in Gaza, Israel did not give the U.S. a heads-up on Israel's mass evacuation order for northern Gaza, according to a US official.

"Of course, we're concerned about any additional loss of innocent human life," said John Kirby, spokesperson for the National Security Council.

"We are working very aggressively with the Israelis and the Egyptians to try to find a safe passage out of southern Gaza," he said.

-ABC News' Mary Bruce, Shannon Crawford and Justin Fishel

The World Health Organization is begging Israel to immediately reverse its evacuation order for the over 1 million living north of Wadi Gaza, saying "a mass evacuation would be disastrous for patients, health workers and other civilians left behind or caught in the mass movement."

"With ongoing airstrikes and closed borders, civilians have no safe place to go," the WHO said in a statement. "Almost half of the population of Gaza is under 18 years of age. With dwindling supplies of safe food, clean water, health services, and without adequate shelter, children and adults, including the elderly, will all be at heightened risk of disease."

"Of the thousands of patients with injuries and other conditions receiving care in hospitals, there are hundreds that are severely wounded and over 100 who require critical care," the WHO said. "Many thousands more, also with wounds or other health needs, cannot access any kind of care. The compressed timeframe, complex transport logistics, damaged roads, and, above all, lack of supportive care during transport all add to the difficulty of moving them."

At a press conference with the Qatari prime minister, Secretary of State Antony Blinken discussed efforts to implement safe zones in Gaza and provide other humanitarian assistance, but did not shed additional light on efforts to evacuate Americans and other foreign nationals ahead of a potential Israeli ground incursion.

"When it comes to providing for civilians in Gaza, both ensuring that they can be out of harm's way and that they can have access to the support that they need -- the humanitarian assistance the food, the medicine, water -- our focus now is on helping to create safe zones. And we're doing that with leading international organizations, and we're doing that engaged with Israel," Blinken said.

But Blinken said that those efforts to help Palestinians were still being actively hindered by Hamas.

"Hamas continues to use innocent civilians as human shields," he said.

"We know the humanitarian situation is urgent. We're actively engaged with partners, including Qatar, to get aid to the those who need it," he said.

-ABC News' Shannon Crawford

Human Rights Watch has accused Israel of using white phosphorus munitions in military operations in Gaza on Wednesday, saying it violated international humanitarian law by putting civilians at unnecessary risk.

In a report released Thursday, Human Rights Watch said it based its determination on a video it said it has verified, as well as two eyewitness accounts.

The Israel Defense Forces said in a statement to ABC News that claims they used white phosphorus in Gaza are "unequivocally false."

ABC News geolocated a video showing a jellyfish-shaped plume of white smoke over the port of Gaza. The video was posted on Wednesday by the official Palestinian news agency WAFA and matches images posted on social media and by other news outlets on the day.

Two experts -- Amael Kotlarski, the weapons team manager for the defense intelligence firm Janes, and Amnesty International weapons investigator Brian Castner -- told ABC News that images suggested munitions deployed over the port of Gaza on Wednesday appeared to contain white phosphorus.

"We have verified that Israeli artillery forces striking Gaza are equipped with M825 and M825A1155mm white phosphorus projectiles," Castner told ABC News, pointing to photographs by an Anadolu Agency photographer taken on Monday and showing Israeli troops handling shells labeled "M825A1."

Kotlarski said visual evidence alone was not sufficient to confirm that these specific shells were used in the Gaza port strikes.

There are "perfectly legal" military uses for white phosphorus munitions, Kotlarski said, including to create a smoke screen to conceal movements, to mark targets for artillery or air strikes, or to illuminate a battlefield. However, its use in urban areas is "problematic due to the higher risk of collateral damage," Kotlarski said, including the munitions' potential to cause severe burns.

White House Principal Deputy Press Secretary Olivia Dalton said Friday the U.S. had not independently verified allegations Israel used white phosphorous "so that is not something I would want to speculate about or weigh in on at this time."

-ABC News' Christopher Looft, Ben Gittleson and Jordana Miller