The U.S. will soon have the capability to move thousands of American citizens and their families out of Israel by sea, two sources familiar with the planning told ABC News.

Those fleeing by sea will be taken to Cyprus aboard a cruise ship-like vessel set to depart from the northern Israeli port city of Haifa on Monday, the sources said.

Boarding is scheduled to begin at 8 a.m. local time on Monday, according to the U.S. State Department. Boarding will proceed in order of arrival as capacity permits.

Flight departures out of Israel’s Ben Gurion International Airport will also continue "on a rolling basis" on Monday and Tuesday, according to the State Department.

"Please go to Ben Gurion International Airport, Terminal 3, if you wish to depart Israel," the State Department instructed travelers in an advisory on Sunday. "U.S. Embassy personnel will be present to direct you and provide specific flight information. Be prepared to wait."

-ABC News' Mary Alice Parks and Shannon Crawford

The number of U.S. citizens killed in the Hamas attack on Israel rose to 30 on Sunday, according to the U.S. State Department.

At least 13 American nationals are still missing, a State Department spokesperson said.

The death toll for Americans killed in Israel rose from 29 on Saturday.

"At this time, we can confirm the deaths of 30 U.S. citizens. We extend our deepest condolences to the victims and to the families of all those affected," the State Department spokesperson said. "At this time, we are also aware of 13 American nationals who are unaccounted for. State Department personnel have been in contact with their families.

"The U.S. government is working around the clock to determine their whereabouts and is working with the Israeli government on every aspect of the hostage crisis, including sharing intelligence and deploying experts from across the United States government to advise the Israeli government on hostage recovery efforts."

-ABC News' Shannon Crawford

With deaths mounting from Israeli airstrikes in Gaza, the head of the U.N. Relief and Work Agency said Sunday that Gaza is running out of body bags.

Philippe Lazzarini, the UNRWA commissioner-general, called a news conference Sunday to "raise the alarm" that as of Sunday he and his colleague are "no longer able to provide humanitarian assistance" in Gaza.

"In fact, Gaza is being strangled and it seems that the world right now has lost its humanity," Lazarrini said.

He said Gaza has no electricity and is running out of water fast.

He added that an "unprecedented humanitarian catastrophe is unfolding under our eyes" in Gaza.

"Thousands of people have been killed, including children and women.  Gaza is now even running out of body bags," said Lazarrini.

The government media office in Gaza said is now considering digging mass graves, according to Reuters.

Lazarrini said 14 staff members of his agency are among the dead.

"They were teachers, engineers, guards and psychologists, an engineer and a gynecologist," Lazzarini said. "Most of our 13,000 UNRWA staff in the Gaza Strip are now displaced or out of their homes."

The number of hostages being held by Hamas in Gaza is now believed to be 155, Israel Defense Forces said Sunday.

The number of hostages rose from 126 earlier Sunday, the IDF said. An undetermined number of Americans are believed to be among the hostages, according to U.S. officials.

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