Four U.S. troops were wounded Saturday during an aborted air mission to evacuate American citizens amid clashes between warring factions in South Sudan, according to the Pentagon.

Three U.S. military aircraft were fired upon from the ground as they approached the town of Bor to rescue U.S. citizens, the Pentagon said in a statement. Bor, the capital of the central state of Jonglei, was seized this week by South Sudanese rebels.

Four U.S. service personnel were injured by the gunfire, and all three aircraft sustained damage, Pentagon officials said. After being hit, the tilt-rotor CV-22 Ospreys — which take off and hover like a helicopter but fly straight ahead like an airplane — veered away from Bor and were diverted to Entebbe, Uganda, the Pentagon said.

A U.S. Air Force C-17 then flew the four troops to Nairobi, the Kenyan capital, for medical treatment, according to the statement. It said the four were listed in stable condition.

Earlier Saturday, Jonathan Dahm, a spokesman for the U.S. Africa Command in Stuttgart, Germany, had said that one of the service members was in “fairly serious” condition and that the three others were being treated for injuries that were not life threatening.

Neither Dahm nor the Pentagon would identify which branch of the armed services the troops belonged to.

In tweets Saturday, Toby Lanzer, a senior U.N. official in the capital, Juba, described the situation in Bor as “tense.”

“We’ve heard clashes & seen bodies in the streets. Civilians have left town to flee for their safety,” he wrote.

Lanzer, who is the deputy special representative of the U.N. secretary general in the U.N. mission in South Sudan, added that more than 15,000 people have sought refuge at the U.N. base in Bor, including Americans, British, Canadians and others.

On Wednesday, President Obama ordered the deployment of 45 U.S. military personnel to South Sudan to protect the U.S. Embassy in Juba and Americans there.

The troops wounded Saturday, however, were part of another group sent to South Sudan specifically for the attempted evacuation in Bor, said Army Col. Thomas Davis, a spokesman for the U.S. Africa Command. He declined to say how many forces were aboard the three Ospreys or where their flights originated from.

South Sudan separated from Sudan to become a new nation in 2011 after a U.S.-backed peace process and a long civil war. The United States and its allies have injected billions of dollars into the fledgling country. But its future has been severely tested by ethnic strife that broke out anew a week ago, killing more than 500 people, according to the United Nations.

The fighting has pitted South Sudanese troops loyal to President Salva Kiir against followers of his former vice president, Riek Machar, who was fired in July. The violence has escalated since Kiir claimed that Machar and his loyalists had staged an attempted military coup last Sunday. Kiir is a member of the majority Dinka tribe, while Machar belongs to the Nuer ethnic group.

The attack on the U.S. aircraft came two days after more than 2,000 ethnic Nuer youths attacked a U.N. base in the remote town of Akobo, in Jonglei state. The assailants killed at least 11 ethnic Dinka seeking refuge at the base, as well as two U.N. peacekeepers from India, the United Nations said.

Saturday’s attacks were the latest sign that the world’s newest nation could be spiraling toward a civil conflict fueled by a turbulent cocktail of ethnic, tribal and political divisions that has roiled the country in the past two years.

The Obama administration has signaled growing concern about the crisis. The referendum that led to the creation of South Sudan is widely considered to be one of its few successes in sub-
Saharan Africa, and Secretary of State John F. Kerry announced Friday that he is sending a special envoy, Ambassador Donald Booth, to the country.

Kerry, in a statement Friday, urged South Sudan’s leaders to “rein in armed groups under their control, immediately cease attacks on civilians, and end the chain of retributive violence between different ethnic and political groups. The violence must stop, the dialogue must intensify.”

In an audio message Friday, national security adviser Susan E. Rice urged the people of South Sudan to “make the choice for peace, make the choice for a unified and cohesive South Sudan.”

At the Pentagon, Defense Secretary Chuck Hagel received several updates on the situation and was reviewing further options from Army. Gen. David Rodriguez, the commander of the Africa Command, a U.S. defense official said.

As many as 40,000 civilians have swarmed U.N. peacekeeping bases in Juba and Bor seeking refuge. The U.N. Security Council, after an emergency session Friday, declared “grave alarm” at the fighting.

Hilde F. Johnson, the top U.N. official in South Sudan, issued a statement denouncing the killing of the Dinka civilians and the U.N. peacekeepers, who she said “were here to protect civilians and serve the people of South Sudan.”

The Nuer youths overran the base in Akobo and seized weapons and ammunitions before the peacekeepers and soldiers of the Sudan People’s Liberation Army retook control a few hours later, the United Nations said.

The State Department has suspended normal operations at the U.S. Embassy in Juba. The embassy said in a statement Saturday that the U.S. government has evacuated 450 Americans and citizens of other countries from South Sudan this week. U.S. officials had hoped to begin evacuation flights from Bor on Saturday, but those operations were suspended after the Ospreys were fired on.

On Friday, a U.N. helicopter was fired upon in another part of Jonglei state and was forced to make an emergency landing, according to U.N. officials. The helicopter was one of four dispatched to evacuate 40 peacekeepers from the U.N. base in the town of Yuai. The crew and passengers of the helicopter were later flown to the U.N. compound in the Upper Nile state capital, Malakal. There were no casualties during the operation.

On Saturday evening, the United Nations held a memorial service in Juba for the slain peacekeepers.

“Had it not been for their bravery, the death toll at the [U.N. base] could have been higher,” Johnson said in a statement. “This horrendous act will not stop us from carrying out our work. To anyone who wants to threaten us, attack us or put obstacles in our way, our message remains loud and clear: We will not be intimidated.”

Raghavan reported from Nairobi.