In an interview with ABC News Live, Michael Oren, the former Israeli ambassador to the U.S., warned that we are just at the "beginning of this war."

Oren said the possibility of another front is "almost certain now," saying he thinks Hezbollah will join the conflict. He also warned that this war will differ from past ones in the region like the Six Day War and Yom Kippur War because this will be fought in locations that are densely populated with civilians.

"This war, by very marked contrast, is fought in the Israeli heartland," Oren said. "It is the vast majority of our casualties are civilians. We're fighting against an enemy that wants to destroy us at any cost and will do so in the most brutal manner and massacre our civilians."

Oren said the support from the U.S. has been crucial during a time of great "agony."

"To have the leaders of the most powerful country in the world, to come out in support of us in that way, so categorically, so unequivocally condemning terror was very moving for all of us, a great source of relief," Oren said.

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

While on a bipartisan congressional delegation trip to Israel, Senate Majority Leader Chuck Schumer and other members of the delegation were rushed into a bomb shelter in Tel Aviv to wait out a rocket attack, according to a social media post from Schumer.

"It shows you what Israelis have to go through," Schumer said in his post on X, which included a photo showing him and Republican Sen. Mitt Romney, a Romney staff and several others huddled in the small bomb shelter.