When asked about the possibility of another front opening in the war, Iranian Foreign Minister Hossein Amir-Abdollahian told reporters that "anything is possible."

"Our clear-cut response about future possibilities is that everything is dependent on the moves of the Zionist regime in Gaza," Amir-Abdollahian said at the airport in Beirut.

Following Israel's siege of Gaza, including cutting off its water supply and power, and the death of Palestinian civilians, "anything is possible, and we may witness new events in the region," he said.

Iran, which does not recognize Israel, has acknowledged giving financial and moral support to Hamas.

The Israeli government has issued a warning for Israelis around the world to be vigilant ahead of Friday, when Hamas is calling on their supporters to hold worldwide "Day of Rage" protests that could turn violent.

In the U.S., every major city police department is on a heightened state of alert.

In New York City, the police department has canceled vacations and instructed all members to show up in uniform.

The FBI also said it is "aware" of open-source reporting about the global call for action by Hamas.

"We are working closely with our law enforcement partners across the country to share information and identify and disrupt any threats that may emerge," the FBI said in a statement.

The U.S. Embassy said it'll offer transportation for Americans looking to leaving Israel beginning on Friday.

"It will take some period of time to schedule everyone seeking to depart," the embassy said. "Transportation will be by air to Athens or Frankfurt, or sea from Haifa to Cyprus. You will not be able to choose your destination – we will assign you to the next available flight or ship."

"You should be prepared to depart within 8-12 hours of receiving notice of your booking. Each traveler may bring one small carry on item no more than 22 pounds (10 kg) and one suitcase no more than 35 pounds (16 kg)," the embassy said.

Delta Air Lines said it's partnering with the government to help set up flights through Athens.

United Airlines said it's adding extra flights with capped fares between Athens and Newark, New Jersey, and will use a larger plane for its Athens to Washington flight. "We’re in close touch with the U.S. State Department as it seeks to add additional options for Americans traveling from the region," United said.

American Airlines also said it is working "closely" with the State Department on its needs.

The U.S. expects "these initial travel options to facilitate the safe departure of thousands of U.S. citizens per week," State Department spokesperson Matthew Miller said in a statement.

A separate official said that the State Department remains in contact with "several thousand" American citizens in Israel, but it’s unclear how many will ultimately decide to leave. Some have already departed.

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

-ABC News' Amanda Maile, Will Gretsky and Shannon Crawford

As Hamas militants launched an unprecedented attack on Israel over the weekend, pillaging, kidnapping and killing civilians across several towns, one of the first targets was the Supernova music festival, held in the Negev desert in southern Israel.

At least 260 people were killed, making it one of the worst civilian casualty incidents in Israel's history.

ABC News talked to survivors and families of the missing, and analyzed and verified witness video and security footage to piece together how the brutal killings unfolded.

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