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.

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