President Joe Biden said he spoke with Jordanian King Abdullah II, members of the U.S. Congress and directed his national security team to remain in contact with their Israeli counterparts.

"I've also directed my team to remain in constant contact with leaders throughout the region including Egypt, Turkey, Qatar, Saudi Arabia, Jordan, Oman, UAE, as well as our European partners and the Palestinian Authority," Biden said.

President Joe Biden delivered remarks from the White House on Saturday expressing U.S. support for Israel in light of Hamas' attack.

"We will not ever fail to have their back," Biden said.

"Israel has the right to defend itself and his people full stop. There is never a justification for terrorist attacks and my administration's support for Israel security is rock solid and unwavering. Let me say this as clearly as I can. This is not a moment for any party hostile to Israel to exploit these attacks to seek advantage. The world is watching," Biden said.

Secretary of State Antony Blinken "reaffirmed" the U.S.' solidarity with Israel during a call with Israeli President Isaac Herzog and Foreign Minister Eli Cohen, a State Department spokesperson said.

"Secretary Blinken reiterated his condolences for the victims of the terrorist attacks against Israel and condemned those attacks in the strongest terms," the spokesperson, Matthew Miller, said in a statement. "Secretary Blinken also discussed measures to bolster Israel’s security. The Secretary underscored the United States' unwavering support for Israel's right to defend itself."

President Joe Biden is set to deliver remarks at 2:30 p.m. on the attacks in Israel from the State Dining Room, according to the White House.

-ABC News' Fritz Farrow

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