President Joe Biden on Friday spoke briefly about his phone call with family members of missing Americans.

"We're working around the clock to secure the release of Americans held by Hamas," Biden said.

"They're going through agony not knowing what the status of their sons, daughters, husbands, wives, children are," Biden said. "It's gut-wrenching. I assured them my personal commitment to do everything possible, everything possible, to return every missing American to their families."

-ABC News' Molly Nagle

UNICEF on Friday urged Israel to rescind its call for a mass evacuation of northern Gaza, saying there needs to be an immediate humanitarian pause to "scale and sustain lifesaving services for children."

"The situation is catastrophic, with unrelenting bombing and a massive increase in the displacement of children and families. There are no safe places," UNICEF Executive Director Catherine Russell said in a statement. "An immediate ceasefire and humanitarian access are the top priorities to allow much needed aid to children and families in Gaza."

"There are rules of war," the statement continued. "Children in Gaza need lifesaving support and every minute counts.”

UNICEF said the situation is becoming "increasingly difficult and dangerous" and humanitarians have been warned to leave Gaza City, but that it will stay to provide support for children in need.

-ABC News' Ben Gittleson

Lufthansa is suspending flights to and from Beirut, Lebanon, "due to the current situation in the Middle East," the airline said in a statement on Friday.

Lufthansa and Eurowings flights to the region will be suspended through Oct. 16.

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

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