New Jersey Sen. Cory Booker arrived in Israel on Friday for several days of planned meetings and was in Jerusalem when the attacks began, Maya Krishna-Rogers, a spokesperson, said in a statement Sunday.

He sheltered in place with his staff and safely departed the country earlier Sunday, according to the statement.

Booker was in Israel for "planned meetings and site visits ahead of the start of an Abraham Accords-focused N7 summit on regional economic integration in Tel Aviv, at which he was scheduled to speak on Tuesday," the statement read.

-ABC News' Mariam Khan

The U.N. agency for Palestinian Refugees said more than 20,000 people are sheltering in schools around Gaza.

-ABC News' Joe Simonetti

The IDF said it's evacuating Israeli civilians from at least two dozen towns along the Gaza border within the next 24 hours.

-ABC News' Joe Simonetti

The U.S. is sending a carrier strike group to the eastern Mediterranean to bolster deterrence in the region and "will be rapidly providing the Israel Defense Forces with additional equipment and resources, including munitions," according to a statement from Defense Secretary Lloyd Austin on Sunday.

"Today, in response to this Hamas attack on Israel, and following detailed discussions with President Biden, I have directed several steps to strengthen Department of Defense posture in the region to bolster regional deterrence efforts," he said.

The U.S. materiel support will begin shipping today and will arrive in the coming days, according to Austin.

The strike group will include the USS Gerald R. Ford aircraft carrier, the guided-missile cruiser USS Normandy and four guided-missile destroyers -- the USS Thomas Hudner, USS Ramage, USS Carney and USS Roosevelt, Austin said.

The U.S. will also boost its F-35, F-15, F-16 and A-10 fighter squadrons in the region, according to the defense secretary.

"The U.S. maintains ready forces globally to further reinforce this deterrence posture if required," he said.

-ABC News' Matt Seyler

The United States is warning against other countries from becoming involved in the Israel-Gaza conflict amid fears that it could spread into a wider war in the Middle East.

"This is not the moment for other parties who are hostile to Israel to seek advantage or to seek to exploit the attacks that have taken place," Jon Finer, deputy national security adviser to U.S. President Joe Biden, Monday on "Good Morning America."

"That is part of why the United States has moved the Carrier Strike Group to the eastern Mediterranean to send a strong and unmistakeable signal that no one else should get involved in this," he added. "We'll see how things unfold in the course of the coming days."

Finer also emphasized that the U.S. believes "Israel has every right to defend itself full-stop."

"Israel will ultimately make the decisions about how it chooses to go about and conduct that defense," he said. "We are offering support in a number of ways."

Finer noted that "more U.S. steps to show support and solidarity for Israel" can be expected.

"The U.S. is going to offer support at every level for Israel's defense and we are working through details those details with our intelligence professionals, our military and our diplomats," he added. "We are in daily -- in fact, many times a day -- contact with Israeli counterparts to see what they need and offer it."

The U.S. "strongly suspect[s] that there will be American citizens among those killed" in the ongoing conflict, according to Finer.

"We are looking obviously very intensively into whether there were any Americans were among those who have been abducted and brought to Gaza. This is still ongoing," he said. "There is still fighting inside Israel as we speak and there is a bit of fog of war in terms of the ability to gather specific information. And so, we will have much more to say about this at the right time."

When asked for comment on a recent report by The Wall Street Journal that Iran was behind Hamas' latest attack on Israel, Finer responded: "We have no direct information to confirm that report. We've obviously seen it, we're looking into it, but we do not have the ability to corroborate it at this time."

"What we can be quite clear about is that Iran is broadly complicit in these attacks for having supported Hamas going back decades -- for having provided financial support, for having provided training, for having provided weapons to Hamas," he added. "What we don't have is direct information that shows Iranian involvement in ordering or planning the attacks that took place over the last couple days. It's something that we’re going to keep looking at closely."