Senate Majority Leader Chuck Schumer on Saturday spoke with top officials from the Central Intelligence Agency and the State Department during a briefing on the situation in Israel.

Schumer's office released a statement, stating, "The enormous loss of life is gut-wrenching, and I am appalled at the viciousness of the attack by Hamas on innocent Israeli civilians. I expressed my commitment to top Biden Administration officials – and directly to Ambassador Herzog – to do all I can to deliver everything Israel requires in this time of urgent need. I will continue to receive regular briefings."

Schumer is currently overseas in China on a Congressional trip.

The statement continued, "This is one of the worst attacks Israel has ever experienced, and friends of Israel in the US and around the world must stand strongly by her. My heart goes out to those that lost loved ones."

The Federal Aviation Administration issued a statement Saturday night urging U.S. airlines and pilots to "use caution" when flying in Israeli airspace.

In a Notice to Air Missions (NOTAM), the FAA said to exercise extreme caution at all altitudes due to the "ongoing conflict situation between Israel and Gaza."

NOTAM's statement told flight crews to remain in contact with "air traffic control at all times" and to monitor for "possible airspace closures."

A music festival near Re'im, Israel, came under fire amid the Hamas attacks, a festivalgoer recounted to ABC News.

Eliav Klein said that around 5:30 a.m. local time Saturday, missiles and gunfire began to target the festival. He and others ran "in a panic," losing track of many of their friends. Some of Klein's friends are still missing, he said.

Footage captured rockets flying across the horizon, interrupting the festival.

-ABC News' Christopher Looft

A senior White House official told reporters Saturday evening that it is "too early" to know what involvement, if any, Iran had in the attacks in Israel.

"It's too early to say whether the state of Iran was directly involved or planning, supporting," the senior administration official said. "We are going to be looking at that very closely."

"That said, there's no doubt Hamas is funded, equipped, armed by Iran and others," the official added.

Several Israeli officials, including President Isaac Herzog, have claimed that Iran helped coordinate the attacks.

The White House official said President Joe Biden and the administration have been in contact with Palestinian and Israeli officials throughout the day.

"We are in deep talks with the Israelis about some of their particular needs," the official said.

-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