The union representing American Airlines pilots told its members to "cease flight operations to Israel" amid conflict in the region.

The Allied Pilots Association (APA) president, Ed Sicher, told members to refuse assignments into Israel "until we can be reasonably assured of the region's safety and security."

American Airlines told ABC News earlier Sunday it canceled flight AA145 from Ben Gurion International Airport in Tel Aviv to John F. Kennedy International Airport in New York scheduled for Monday. The carrier said it continues to monitor the situation in Israel and will make further adjustments as needed.

American had announced Saturday it was temporarily suspending operations to and from Tel Aviv and would continue to monitor the situation.

-ABC News' Sam Sweeney, Amanda Maile

Vice President Kamala Harris spoke with Israeli President Isaac Herzog on Sunday to express her and second gentleman Doug Emhoff's condolences and stress U.S. commitment to Israel's security, according to her office.

-ABC News' Fritz Farrow

New York Rep. Dan Goldman and his family were in Israel for a bar mitzvah and on Saturday "sheltered from Hamas rocket fire in their hotel's interiors stairwell until early Sunday," a spokesperson, Simone Kanter, said in a statement.

Goldman and his family were able to safely depart Israel with the help of the State Department and Israeli officials, his spokesperson said.

-ABC News' Lauren Peller

Hospitals in Gaza are "becoming overwhelmed" and are "overcrowded with injured people," Doctors Without Borders/Médecins Sans Frontières said in a statement Sunday.

They're facing a shortage of drugs, medical supplies and fuel for generators, according to MSF, as well as the challenge of safely transporting patients to facilities.

"Ambulances can't be used right now because they're being hit by airstrikes," Darwin Diaz, MSF medical coordinator in Gaza, said.

-ABC News' Cindy Smith

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