NAIROBI, Kenya - Gunfire hit three U.S. military aircraft trying to evacuate American citizens in a remote region of South Sudan that on Saturday became a battleground between the country's military and renegade troops, officials said. Four U.S. service members were wounded in the attack in the same region where gunfire downed a U.N. helicopter the day before.

The U.S. military aircraft were about to land in Bor, the capital of the state of Jonglei and scene of some of the nation's worst violence over the last week, when they were hit. The military said the four wounded troops were in stable condition.

The U.S. military said three CV-22 Ospreys - tilt-rotor aircraft that can fly like both a helicopter and a plane - were "participating in a mission to evacuate American citizens in Bor." A South Sudan official said violence against civilians there has resulted in bodies "sprinkled all over town."

"After receiving fire from the ground while approaching the site, the aircraft diverted to an airfield outside the country and aborted the mission," the statement said. "The injured troops are being treated for their wounds." It was not known how many U.S. civilians are in Bor.

Injured flown out

The aircraft flew to Entebbe, Uganda. From there, the injured were flown to Nairobi, Kenya, aboard a U.S. Air Force C-17 for medical treatment, the statement said.

An official in the region who insisted on anonymity to share information not made public said the Americans did not tell the top commander in Bor - Gen. Peter Gadet, who defected from the South Sudan military this week - that they were coming in, which may have led to the attack. The U.S. statements said the gunfire was from unknown forces.

South Sudan's military spokesman, Col. Philip Aguer, said that government troops were not in control of Bor, so the attack on the U.S. aircraft had to be blamed on renegade soldiers.

"Bor is under the control of the forces of Riek Machar," Aguer said, referring to the country's ousted vice president.

The U.S. Embassy in Juba said it had evacuated at least 450 Americans and other foreign nationals from Juba this week and had hoped to begin evacuations from Bor. The Ospreys were hit one day after small-arms fire downed a U.N. helicopter in the same state.

Peacekeepers exit

The U.N. on Friday sent four helicopters to extract 40 U.N. peacekeepers from a base in Yuai, also in Jonglei, U.N. information officer Joe Contreras said. One helicopter was fired on and made an emergency landing in Upper Nile state, he said. No casualties occurred during the incident.

South Sudan's information minister, Michael Makuei Lueth, said that South Sudanese ground troops, backed by the country's air force, are fighting rebels in Bor in an effort to retake the state capital they lost earlier this week.

"There is fighting going on in Bor town, yes, because since morning they have continued to attack the civilian population," Lueth said, talking about renegade troops. "They have gone as far as not respecting the U.N. compound."

He said fighting started early Saturday after reports came in that rebels there were shooting indiscriminately at civilians.

"The bodies are sprinkled all over the town," he said. No death toll could be estimated, he said.

South Sudan President Salva Kiir, an ethnic Dinka, said last week that an attempted coup triggered the violence now pulsing through the country. He blamed the former vice president, Machar, an ethnic Nuer. But officials have since said a fight between Dinka and Nuer members of the presidential guard triggered the initial violence late Sunday.