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.

The U.S. Embassy said it'll offer transportation for Americans looking to leaving Israel beginning on Friday.

"It will take some period of time to schedule everyone seeking to depart," the embassy said. "Transportation will be by air to Athens or Frankfurt, or sea from Haifa to Cyprus. You will not be able to choose your destination – we will assign you to the next available flight or ship."

"You should be prepared to depart within 8-12 hours of receiving notice of your booking. Each traveler may bring one small carry on item no more than 22 pounds (10 kg) and one suitcase no more than 35 pounds (16 kg)," the embassy said.

Delta Air Lines said it's partnering with the government to help set up flights through Athens.

United Airlines said it's adding extra flights with capped fares between Athens and Newark, New Jersey, and will use a larger plane for its Athens to Washington flight. "We’re in close touch with the U.S. State Department as it seeks to add additional options for Americans traveling from the region," United said.

American Airlines also said it is working "closely" with the State Department on its needs.

The U.S. expects "these initial travel options to facilitate the safe departure of thousands of U.S. citizens per week," State Department spokesperson Matthew Miller said in a statement.

A separate official said that the State Department remains in contact with "several thousand" American citizens in Israel, but it’s unclear how many will ultimately decide to leave. Some have already departed.

The State Department estimates that roughly 500,000 American citizens live in Israel.

-ABC News' Amanda Maile, Will Gretsky and Shannon Crawford