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

National Security Council spokesperson John Kirby avoided saying if the U.S. believed Israel's calls for civilians in Gaza to evacuate in the next 24 hours was possible but acknowledge that it was a "tall order" given how densely populated the area is.

"Well, I think we're going to be careful not to get into armchair quarterbacking the tactics on the ground by the IDF. What I can tell you is we understand what they're trying to do. They're trying to move civilians out of harm's way and giving them fair warning. Now, it's a tall order. It's a million people, and it's a very urban, dense environment, already a combat zone. I don't think anybody's underestimating the challenge here of affecting that evacuation," Kirby said in an interview on CNN Friday morning.

"But it's pretty clear that what they're trying to do is to the maximum extent possible avoid civilian casualties and also separate Hamas from the human shields. I mean, Hamas actually gave a counterorder telling Palestinians in Gaza to stay at home. Why? Because having human shields, they think, protects them," Kirby continued.

Asked about their efforts to secure a safe passage for civilians out of Gaza, Kirby reiterated that the U.S. is talking to Israel and Egypt but would not comment further on progress.

On the Rafah crossing specifically, Kirby said "So far it's still closed. But that doesn't mean were not going to still try as hard as we can to talk to the Israelis and Egyptians about getting it open and having some sort of safe passage out. It's important that we also continue to work to make sure that humanitarian assistance can get in. The United States has no intention of stopping our efforts to continue to deliver humanitarian assistance, but there has to be a path in, there has to be a path out."

Kirby had no update on the Americans who are believed to be held hostage in Gaza and could not confirm reports that 13 hostages have been killed by Israeli airstrikes, as Hamas has claimed.

-ABC News' Molly Nagle