At least 50 Israelis are currently being held hostage by Hamas, according to Israeli Command. That number may change as Israeli forces get a handle on the situation on the ground.

During a call with Palestinian Authority President Mahmoud Abbas, Secretary of State Antony Blinken called on the region's leadership to "condemn" the Hamas attacks, according to a readout from the State Department.

"The Secretary reiterated the United States' unequivocal condemnation of the terrorist attacks by Hamas against Israel, and called on all leadership in the region to condemn them," State Department spokesperson Matthew Miller said in a statement. "The Secretary urged the Palestinian Authority to continue and enhance steps to restore calm and stability in the West Bank."

Earlier Saturday, Abbas said his people have the right to defend themselves against the "terror of settlers and occupation troops," according to Reuters.

In a strongly worded statement, Israel's ambassador to the U.S. labeled the Hamas attacks as "war crimes" and vowed that Israel will fight back.

"Those of our enemies who believe that Israel is weak because of its internal debate have totally miscalculated. When under attack, Israelis close ranks and join together in fighting for Israel's self-defense," Michael Herzog declared. "This is war. We will fight to win and deter the terrorists from any future attacks."

The ambassador also directly blamed Iran for the surprise attack, saying, "Hamas is a U.S. and E.U. designated terror organization and a close ally of Iran. Iran's hands are evidently behind the scenes, leading the so-called Axis of Resistance to Israel's existence."

-ABC News' Shannon K. Crawford

Prime Minister Netanyahu said Israel will "reach every place Hamas is hiding" during an address late Saturday and urged "Gaza's people to leave those places now.”

Israel will take "vengeance for this black day," he added.

At least 300 people have been killed and thousands injured since Hamas launched its surprise attack. The Palestinian Health Ministry said 232 are dead and another 1,790 injured in Gaza. According to the Israeli Health Ministry, over 100 people are dead and over 900 others are injured -- though multiple Israeli news outlets are reporting at least 250 Israelis have been killed and another 1,500 injured.

Israeli army spokesman Richard Hech also reported "severe" hostage situations were ongoing in Kibbutz Be'eri and Ofakim, with an unknown number of Israelis being held hostage.

Israel said the assault started at sunrise, when Hamas attacked 22 sites bordering Gaza by breaching some border fences.

The United States is warning against other countries from becoming involved in the Israel-Gaza conflict amid fears that it could spread into a wider war in the Middle East.

"This is not the moment for other parties who are hostile to Israel to seek advantage or to seek to exploit the attacks that have taken place," Jon Finer, deputy national security adviser to U.S. President Joe Biden, Monday on "Good Morning America."

"That is part of why the United States has moved the Carrier Strike Group to the eastern Mediterranean to send a strong and unmistakeable signal that no one else should get involved in this," he added. "We'll see how things unfold in the course of the coming days."

Finer also emphasized that the U.S. believes "Israel has every right to defend itself full-stop."

"Israel will ultimately make the decisions about how it chooses to go about and conduct that defense," he said. "We are offering support in a number of ways."

Finer noted that "more U.S. steps to show support and solidarity for Israel" can be expected.

"The U.S. is going to offer support at every level for Israel's defense and we are working through details those details with our intelligence professionals, our military and our diplomats," he added. "We are in daily -- in fact, many times a day -- contact with Israeli counterparts to see what they need and offer it."

The U.S. "strongly suspect[s] that there will be American citizens among those killed" in the ongoing conflict, according to Finer.

"We are looking obviously very intensively into whether there were any Americans were among those who have been abducted and brought to Gaza. This is still ongoing," he said. "There is still fighting inside Israel as we speak and there is a bit of fog of war in terms of the ability to gather specific information. And so, we will have much more to say about this at the right time."

When asked for comment on a recent report by The Wall Street Journal that Iran was behind Hamas' latest attack on Israel, Finer responded: "We have no direct information to confirm that report. We've obviously seen it, we're looking into it, but we do not have the ability to corroborate it at this time."

"What we can be quite clear about is that Iran is broadly complicit in these attacks for having supported Hamas going back decades -- for having provided financial support, for having provided training, for having provided weapons to Hamas," he added. "What we don't have is direct information that shows Iranian involvement in ordering or planning the attacks that took place over the last couple days. It's something that we’re going to keep looking at closely."