At least 32 members of Congress wrote a letter to the CEOs of American Airlines, Delta Air Lines and United Airlines urging them "to resume flights as soon as possible to Tel Aviv and keep your operations flexible to assist with bringing Americans home and moving innocent civilians out of harm's way."

While U.S.-based airlines have stopped flights to and from Tel Aviv, they have been working with the U.S. government to bolster their operations in Europe to help get fleeing Americans home after they land in European cities.

The lawmakers wrote that certain "barriers" to resuming operations "may seem too complex or too difficult, but as long as there are Americans in need and a way to operate safely -- we must try."

-ABC News' Ben Gittleson

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."

As time dwindles ahead of an anticipated Israeli ground invasion of Gaza, the U.S. is calling on fleeing citizens to head toward the southern border crossing to Egypt.

"We have been working around the clock. We have an entire dedicated team that is working on nothing but this: helping American citizens who are in Gaza be able to get safe passage through the border crossing to Egypt," Jake Sullivan told ABC "This Week" co-anchor Martha Raddatz on Sunday.

The U.S. Embassy in Jerusalem on Sunday morning urged U.S. citizens trapped in the Gaza Strip to move to southern Gaza along the main highway, the Salah al-Din Street.