The U.S. Embassy in Israel issued a security alert on Saturday warning Americans in Gaza seeking to flee to check the status of the Rafah Crossing into Egypt and to remain vigilant.

"U.S. citizens are reminded to remain vigilant and take appropriate steps to increase their security awareness as security incidents, including mortar and rocket fire, often take place without warning. U.S. citizens in Gaza who wish to leave and can do so safely are advised to check the status of the Rafah Crossing into Egypt," the embassy alerted.

"U.S. Embassy personnel are still currently sheltering in place. U.S. government personnel continue to be prohibited from travel to Gaza and areas within seven miles of Gaza," the embassy said.

-ABC News' Fritz Farrow and MaryAlice Parks

The United Nations will hold a private meeting on Sunday to discuss the unfolding Israeli-Palestinian crisis.

The death toll continues to climb with over 300 people reported dead in Gaza and Israel.

The Palestinian Health Authority said 198 are dead and 1,610 others are injured in Gaza. According to the Israeli Health Ministry over 100 people are dead and over 900 others are injured.

Just before sundown in a western Gaza City, a massive explosion and fireball were reported after Israel warplanes hit a high-rise apartment complex. The Gaza Interior Ministry said the building housed approximately 100 families.

U.S. Defense Secretary Lloyd Austin spoke with Israeli Minister of Defense Yoav Gallant on Saturday to convey his condolences.

"Secretary Austin made clear to Minister Gallant his ironclad support for the Israeli Defense Forces and the Israeli people. He reaffirmed that the Department’s commitment to Israel’s security and its absolute right to defend itself from acts of terrorism is unwavering. Secretary Austin has directed his team to ensure that DoD is closely consulting with all of our Allies and partners who share a commitment to peace and oppose terrorism. Secretary Austin will continue to consult with Minister Gallant in the coming days and weeks to ensure that Israel has the support it needs," the Pentagon said in a statement.

-ABC News' Matt Seyler

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