President Obama had sent 45 US service members to South Sudan to "support the security of US personnel and our embassy," he said Thursday.

South Sudan officials said the attack had been carried out by rebel forces.

WASHINGTON — US aircraft flying into a heavily contested region of South Sudan to evacuate American citizens were attacked Saturday and forced to turn back without completing the mission, US officials said. Four service members were wounded, one seriously.

Previous evacuation flights had been organized in Juba, the capital, but the aborted mission Saturday appeared to be the first into rebel-held territory.

The military sent three CV-22 Ospreys — tilt-rotor aircraft that can fly like an airplane and land like a helicopter — to evacuate US citizens from a UN compound in Bor, the capital of Jonglei state.


The United States has not said how many Americans were there or whether they were in immediate danger.

As the aircraft approached the city, "they were fired on by small-arms fire by unknown forces," the military said in a statement. All three aircraft were damaged.

Afterward, the mission was aborted and the Ospreys flew about 500 miles to Entebbe, Uganda.

One US service member was seriously wounded and taken to Nairobi, where he underwent surgery Saturday evening. Three others were reported to have minor injuries, US officials said.

An official in the region told the Associated Press that the Americans did not tell the top commander in Bor — General Peter Gadet, who defected from the South Sudan military this week — that they were coming in, which may have led to the attack.

The US statements did not identify the attackers.

South Sudan's military spokesman, Colonel Philip Aguer, said that government troops are not in control of Bor, so the attack on the US aircraft has to be blamed on renegade soldiers, the Associated Press reported.


''Bor is under the control of the forces of Riek Machar,'' Aguer said.

The US Embassy in Juba said it has evacuated at least 450 Americans and other foreign nationals from Juba last week and had hoped to begin evacuations from Bor.

The Ospreys were hit one day after small arms fire downed a United Nations helicopter in the same area.

The UN on Friday sent four helicopters to extract 40 of its peacekeepers from a base in Yuai, also in Jonglei.

One helicopter was fired upon and executed an emergency landing in Upper Nile state. No casualties occurred during the incident.

As South Sudan has been racked by attacks, there have been mounting concerns about the plight of 35,000 civilians who have sought sanctuary at UN peacekeeping bases in the country.

Some 14,000 civilians are reported to have sought refuge at a base in Bor that is surrounded by 2,000 armed youths, spurring fears that the base may be overrun and the lives of aid workers there threatened.

US officials had little to say about the plight of the civilians remaining at the UN compound, including Americans.

On Friday night, Secretary of State John Kerry issued a statement saying that he had called Salva Kiir, the president of South Sudan, to urge him to "protect all South Sudanese citizens and work toward reconciliation."

"Now is the time for 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," Kerry said in his statement. "The violence must stop, the dialogue must intensify."


To encourage such efforts, Donald E. Booth, the United States' special envoy for Sudan and South Sudan, left for the region Friday.

Kiir, a member of the Dinka ethnic group, has attributed the violence to an attempted coup by a former vice president, Riek Machar, who is a member of the rival Nuer ethnic group.

Earlier this week, Obama sent troops to protect the US Embassy in Juba. The United States has also organized flights to evacuate its citizens from the capital, as have European nations.

But thousands of civilians remain at risk, as do UN peacekeepers.

The danger was underscored by an attack Thursday in which 2,000 armed youths of Nuer ethnicity overran a base in the town of Akobo, killing Dinka civilians and two Indian peacekeepers who tried to protect them.

Hilde F. Johnson, the top UN official in the country, said in a written statement, "I deplore this unjustified and unwarranted attack on the United Nations Mission base in Akobo, killing peacekeepers that were here to protect civilians and serve the people of South Sudan."