President Joe Biden on Friday morning spoke with family members of the 14 Americans who remain unaccounted for after the attack in Israel, according to the White House.

-ABC News' Molly Nagle

Doctors Without Borders issued a statement Friday calling the Israeli government’s 24-hour notice to leave northern Gaza "outrageous."

"We are talking about more than a million human beings," Doctors Without Borders said in a statement. "'Unprecedented' doesn’t even cover the medical humanitarian impact of this. Gaza is being flattened, thousands of people are dying. This must stop now. We condemn Israel’s demand in the strongest possible terms."

Over 423,000 people have been displaced in Gaza so far, according to the United Nations.

Police in London are upping patrols amid a rise in antisemitism following the Hamas terror attack, the Metropolitan Police said.

"Officers have visited more than 200 schools as well as more than 300 synagogues, mosques and other places of worship," police said. "Some of our communities are telling us they feel very vulnerable and we are working with their leaders, faith groups and others to reassure them and ensure they feel safe."

Police said they're expecting thousands of people to participate in a pro-Palestine march in London on Saturday.

Fighting is ongoing in northern Israel near the Lebanon border, where huge plumes of smoke are visible.

The Israel Defense Forces said there was an explosion Friday at the security fence adjacent to Hanita, Israel, which is along the border. The IDF said it's responding with artillery fire toward Lebanon.

"Additionally, an alert was activated concerning the infiltration of terrorists into the community," the IDF said, and Israeli "soldiers are currently searching the area."

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