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.

1:09

Human Rights Watch says IDF used white phosphorus munitions over Gaza; IDF denies use

A video geolocated by ABC News shows a jellyfish-shaped plume of white smoke over the port of Gaza, which was posted on Oct. 11, by the official Palestinian news agency. Human Rights Watch said it shows the use of white phosphorus; Israel’s military has denied this.

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

Six days after a brutal attack by armed Hamas militants on Israeli civilians, Israel ordered an evacuation of more than 1 million Palestinians in northern Gaza ahead of what could be an unprecedented ground offensive there against Hamas.

The U.S. and other nations have repeatedly warned Israel to follow the "rules of war."

But what are they? And will they prevent civilians from being hurt?

Here's what to know about international humanitarian laws and how they might apply in the Israeli-Hamas conflict in Gaza.

-ABC News' Anne Flaherty

As the Biden administration intensifies efforts to help American nationals leave Israel and Gaza, more than 20,000 U.S. citizens have reached out to the State Department asking for help in the wake of the Hamas attacks, including several hundred seeking to leave Gaza, according to two U.S. officials.

While not all have actively sought assistance with transportation, "many thousands" across Israel have indicated they needed help finding flights out of the country, the officials said.

Government-chartered planes have now started to ferry U.S. citizens from Israel to other countries in the region, though one official said the State Department still cannot share a detailed breakdown on the number of Americans whose departure it has already facilitated or the number of Americans still waiting for help "given this is an unfolding situation."

For Americans in Gaza, the U.S. is still eyeing the Rafah border crossing between Gaza and Egypt as a means of safe passage. Officials say they are very hopeful they will be able to work with Israel and Egypt to establish a corridor for U.S. citizens and other foreign nationals, but that it could take days to make substantial progress.

The Israeli government estimated that between 100 to 130 Americans were living in Gaza earlier this year. An official says the State Department believes hundreds more are visiting -- putting the total number in Gaza potentially between 500 and 600.

The State Department estimates that roughly 500,000 American citizens reside in Israel.

-ABC News' Shannon K. Crawford

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."

11:27

Biden talks economics and Israel in Philadelphia

President Joe Biden said "The United States stands with Israel" and will make sure Israel has what it needs to defend itself.

-ABC News' Molly Nagle