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

Hamas fighters are believed to be "still hiding" in some communities in southern Israel even after the Israeli military took back control, according to Israel Defense Forces spokesman Maj. Doron Spielman.

"We are in control of the communities, but we actually assume that there are Hamas terrorists that are still hiding in these areas, including in the road where we are now," Spielman told ABC News during an interview Monday in Sderot, Israel. "In all these communities, we've seen terrorist come out of hiding. Just yesterday, they took over an ambulance."

"We're still being very, very careful," he added. "This is a war zone with active terrorists that are operating here."

Sderot is a southwestern Israeli city located near the county's border with the Hamas-ruled Gaza Strip. A police station there was stormed by Hamas fighters on Saturday as part of the militant group's wider incursion, leaving at least six Israeli police officers dead. The IDF then bombed the police station with the Hamas fighters inside, demolishing it.

Hamas fighters and other Palestinian militants are "striking strategic locations" along the border fence, allowing them to drive through in pickup trucks, Spielman said. Civilians in some of the local communities have been ordered to evacuate, while others have left voluntarily, according to Spielman.

"We do have a lot of ground troops in this area. We have a lot of tanks. We have, in addition, artillery. And we're prepared forever for anything," he said. "And if it happens, the next stage, it's going to happen here because this is the area that was the flashpoint and this is the area we're going to be presenting ourselves and finishing this battle. We, of course, have no choice but to finish."

When asked about the mounting casualties of Palestinian civilians in Gaza as a result of the IDF's retaliatory airstrikes, Spielman said: "We always try to avoid civilian casualties. However, I would say that this is war and our first priority here is to destroy Hamas."

-ABC News' Angus Hines and Ian Pannell