Up to 50 Israelis who were being held hostage by Hamas have been freed, according to an Israel Defense Forces spokesperson.

They were being held in the dining hall of Kibbutz Be'eri in southern Israel, though the situation has been "neutralized," the IDF spokesperson said.

Officials still believe hostages are being held in Gaza but an exact number hasn't been released.

The FBI and the Department of Homeland Security have issued a "Public Safety Notification" to law enforcement around the U.S., urging them to remain vigilant and report any suspicious activity in the midst of the ongoing situation in Israel.

The notice says the agencies "do not currently have specific intelligence indicating a threat to the United States stemming from the Hamas attacks in Israel" but that "foreign terrorist organizations and their supporters remain committed to attacking the United States within and beyond our borders."

"In recent years, there have been several events and incidents in the United States that were purportedly motivated, at least in part, by the conflict between Israel and Hamas," the notice continued. "These have included the targeting of individuals, houses of worship, and institutions associated with the Jewish and Muslim faiths with acts of physical assault, vandalism, or harassment."

-ABC News' Mike Levine

A high-ranking Israel Defense Forces commander was killed in action Saturday, the military said.

Col. Jonathan Steinberg, 42, the commander of the Nahal Brigade, was killed during a confrontation with Hamas, IDF said.

United Hatzalah, a community-based volunteer EMS organization, said so far it has treated nearly 1,000 people for "various injuries" in southern and central Israel.

The organization said it has also transported an unspecified number of patients to hospitals in the regions via ambulance and helicopter.

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