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.

As Americans inside Gaza wait for the Rafah border crossing between Egypt and Gaza to open, a senior State Department official said that Egypt has informed the U.S. that “there are acute security threats" preventing U.S. officials and others from aiding Americans inside Gaza.

The official added that the State Department has placed a team on the Egypt side of the border "as close to the border as conditions permit."

Egyptian Foreign Minister Sameh Shoukry said Monday that Israel has not allowed the opening of the Rafah border crossing from the Gaza side.

Shoukry said, since the crisis broke out, Egypt "has been seeking to keep the crossing operational and in a way that allows the entry of humanitarian aid."

"Until now, unfortunately, the Israeli government has not taken a position to allow the opening of the crossing from the Gaza side for the entry of aid or the exit of nationals of [other] countries," Shoukry said. "We are ready and Egyptian authorities on the border are ready to get aid in and get nationals of third countries out."

He added that Egypt aims to keep normal operation of the border "for the entry of Palestinians with medical needs or the normal movement between the Strip and Egypt."

-ABC News' Shannon Crawford