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

In remarks with Israeli Prime Minister Benjamin Netanyahu, Secretary of State Antony Blinken said at least 25 Americans had now been confirmed dead in Israel.

Netanyahu described some of the atrocities committed by Hamas.

"Hamas has shown itself to be an enemy of civilization: The massacring of young people in an outdoor music festival, the butchering of entire families, the murder of parents in front of their children and the murder of children in front of their parents. The burning of people alive, the beheadings," he said.

"Hamas should be treated exactly the way ISIS was treated," he said. "They shouldn't be spit out from the community of nations. No leader should meet them. No country should harbor them. And those that do should be sanctioned."

Blinken said he was speaking not only as an American diplomat, but as a Jew, a husband and a father of young children, saying it was impossible for him to look at photos of families killed "and not think of my own children."

"The same time that we've been shocked by the depravity of Hamas, we've also been inspired by the bravery of Israel citizens," he said. "The grandfather who drove over an hour to a kibbutz under siege, armed only with a pistol and rescued his kids and grandkids. The mother who died shielding her teenage son with her body, giving her life to save his -- giving him life for a second time."

Blinken remarked that some U.S. aid had already been delivered, and that more military aid was on his way. He said the administration would work with Congress to meet Israel’s additional needs and that bipartisan support for Israel was "overwhelming."

The secretary called on world leaders to condemn Hamas.

Blinken also reiterated that Israel "has the right -- indeed, the obligation -- to defend itself" but that it was "so important to take every possible precaution to avoid harming civilians."

-ABC News' Shannon Crawford