Brad Campbell and Kim Campbell, center, with some of the orphans they are caring for inside the U.N. base in Malakal, South Sudan, Dec. 29, 2013. (Sudarsan Raghavan/The Washington Post)

It was Christmas morning, and the eight Americans and 10 South ­Sudanese orphans lay huddled under their beds, shivering with fear. Outside, rival soldiers were trying to kill one another. Heavy gunfire exploded in waves, rattling the windows.

Trapped for hours, the group prayed silently for survival.

“We were trying to keep the kids quiet,” Bradley Campbell, a missionary from Omaha, said as he recalled their ordeal Sunday, seated inside a U.N. peacekeeping base not far from their home. “We didn’t want anyone to find us.’’

Along with thousands of South Sudanese, Campbell and his group are among scores of Americans who have found themselves in peril in the world’s newest nation as violent clashes rage between rival factions of the army.

The U.S. government has evacuated more than 400 U.S. officials and private citizens since fighting broke out two weeks ago, a State Department official said. But Campbell and his family and friends, along with the orphans they are caring for, remain in Malakal, inside the base where they found refuge on Christmas night.

There was no U.S. evacuation flight Sunday, even though the United Nations considered the situation calm enough to send several planes into Malakal carrying peacekeeping personnel and supplies.

“We are looking to do anything we can to get people out of Malakal in a way that’s safe and secure,” the State Department official said. “Our priority is always American citizens.’’

American officials have refused to say how many more U.S. citizens in South Sudan are awaiting evacuation. At the base in Malakal, U.N. officials said those waiting for a U.S. flight number about 60, including the Campbells’ group as well as other Americans, Canadians, Australians and New Zealanders.

Campbell, 44, is a former visual artist whose submission was among seven finalists for the World Trade Center memorial to honor those killed on Sept. 11, 2001; the competition attracted more than 5,000 entries from 63 nations.

In 2005, he gave up his career in New York to become a pastor and eventually traveled to work in sub-Saharan Africa. He and his wife, Kim, along with some of their five children, arrived in South Sudan in March 2012 to work with refugee children for Keeping Hope Alive, a Christian ministry based in Charlotte.

In a country that is predominantly Christian, the Campbells started taking care of orphans who had lost parents to conflict or illness, a group that has grown to 10 children, ages 6 through 17.

The violence was triggered by fighting Dec. 15 between soldiers loyal to President Salva Kiir and those aligned with his deputy, Riek Machar. The next day, Kiir accused Machar of plotting a coup, sparking bloodshed that quickly spread to five of South Sudan’s 10 states, killing hundreds and forcing tens of thousands to flee to U.N. bases in several towns.

Inside their house, Bradley Campbell’s two stepdaughters, Cassidy and Katie Talbott, worried. Mattresses had been placed against the bedroom door, their suitcases on top of their beds. Both knew about South Sudan’s history of civil conflict and ethnic and tribal clashes, and they had heard stories about the soldiers’ brutality.

“The house would shake, and when the bullets came through the windows, you are trying to mentally prepare yourself,” said Katie Talbott, 23. “What if the soldiers come inside? What are they going to do? We’ve heard stories about them doing horrible things to people.”

After the gunfire began to wind down in the late afternoon, the group grabbed some clothes and snacks and made their way toward the U.N. base. Eventually they arrived at a back gate where thousands of South Sudanese were trying to enter. When U.N. workers advised the Americans to use another entrance, many of the South Sudanese followed.

“A man tried to hand me his child,’’ Katie Talbott recalled. “He said, ‘Please take my baby with you; please take her to safety.’ ”

Eventually the South Sudanese were allowed to enter.

Inside the base, conditions were dire. Campbell recalled that his group’s only food was packets of nuts and dried fruit brought from their house. Water, too, was scarce, and they depended on the kindness of U.N. workers and others. For the past five nights, Campbell and his group have slept together on a floor with other displaced people — 24 in a single room. On Sunday, Campbell held a church service inside the camp.

Many of the South Sudanese from their community in Malakal are missing, he said.

“We don’t know where our neighbors are,” said Kim Campbell, 54. “It’s definitely the most intense thing I have ever been through.”

As they wait to be evacuated, another huge concern occupies their minds. Will the U.S. government also fly out the orphans?

The children are not American citizens. The Campbells say they have applied with local child welfare authorities to become their legal guardians, and they worry about what will happen if the children remain in South Sudan.

All but one of the orphans is ethnic Nuer, Kim Campbell said, and some of the fighting has unfolded along tribal lines, pitting ethnic Dinka, the group to which the president belongs, against Nuer, the vice president’s group.

“There is no Plan B for them,” she said. “Because they are Nuer, there could be a death sentence on them.”

Bradley and Kim Campbell said they will not leave Malakal without the orphans, but they want to get their daughters out regardless.

Their daughters have a different idea.

“We want to get out. But I am not going to leave unless the kids are going to come,” Katie said as Cassidy, 16, nodded in agreement. “I am not going to leave my parents here, either, knowing what they are going through and what it is like here. I can’t consciously be sitting back in America without them.”

Karen DeYoung in Washington contributed to this report.