More than 700 people are dead in Israel and over 2,100 others injured, Israeli health officials said.

The Palestinian Health Authority said there are 370 people dead in Gaza and 2,200 others injured.

President Biden spoke with Israeli Prime Minister Benjamin Netanyahu on Sunday and informed him the U.S. was providing "additional assistance for the Israeli Defense Forces" was already on its way "with more to follow over the coming days," according to the White House.

-ABC News' Fritz Farrow

President Joe Biden "directed additional support for Israel in the face of this unprecedented terrorist assault by Hamas," the White House said Sunday.

Biden and Vice President Kamala Harris received briefings Sunday morning on the attacks in Israel.

-ABC News' Fritz Farrow

U.S. Secretary of State Antony Blinken on Sunday said the Biden administration is working to verify reports that U.S. citizens are among the dead and missing in Isreal following Saturday's unprecedented attacks from Hamas into Israel from Gaza.

“We have reports that several Americans were killed. We are working overtime to verify that,” Blinken told CNN's "State of the Union. "There are reports of missing Americans."

Blinken called the attack the worst since the 1973 Yom Kippur War, but added that there is a "fundamental difference."

"That was a war that was state-to-state, country-to-country, army-to-army. This is a massive terrorist attack that is gunning down Israeli civilians in their towns and their homes," Blinken said. "As we've seen so graphically, they're literally dragging people across the border with Gaza, including a Holocaust survivor in a wheelchair, women and children. You can imagine the impact this is having throughout Israel and the world should be revolted at what it's seen."

-ABC News' Joe Simonetti

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