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 World Health Organization is begging Israel to immediately reverse its evacuation order for the over 1 million living north of Wadi Gaza, saying "a mass evacuation would be disastrous for patients, health workers and other civilians left behind or caught in the mass movement."

"With ongoing airstrikes and closed borders, civilians have no safe place to go," the WHO said in a statement. "Almost half of the population of Gaza is under 18 years of age. With dwindling supplies of safe food, clean water, health services, and without adequate shelter, children and adults, including the elderly, will all be at heightened risk of disease."

"Of the thousands of patients with injuries and other conditions receiving care in hospitals, there are hundreds that are severely wounded and over 100 who require critical care," the WHO said. "Many thousands more, also with wounds or other health needs, cannot access any kind of care. The compressed timeframe, complex transport logistics, damaged roads, and, above all, lack of supportive care during transport all add to the difficulty of moving them."