SSUDAN-UNREST-USA

The evacuation of Americans from the South Sudan capital, Juba, was being assisted by U.S. military aircraft and personnel. (Samir Bol / AFP/Getty Images / December 21, 2013)

JOHANNESBURG, South Africa — South Sudanese militias fired on two U.S. military aircraft Saturday, wounding four U.S. servicemen, one of them critically, according to American military officials.

The two planes were headed to Bor, north of the capital Juba, the main town in the eastern state of Jonglei, which is under the control of a rebel military faction associated with sacked vice president Riek Machar.

Intense fighting has been going on in the region as South Sudan's army struggles to take back the town.

South Sudan is spiraling dangerously toward civil war, with the army losing its grip on key regions, as the military splits and defectors seize control of key areas.

An army division loyal to Gen. James Koang in the main oil-producing region, Unity state, mutinied Friday. Koang later declared himself governor of the state, according to independent local Radio Tamazuj. Fighting ignited in other parts of the north, the station reported.

The country appears to be splitting along ethnic lines in a confrontation between President Salva Kiir and Machar, whom he dismissed, with the entire cabinet, in July. Kiir, of the Dinka ethnic group, has accused Machar, a Nuer, of launching a coup.

Machar accused Kiir of stirring ethnic tensions and denied the coup accusation, but in recent days Machar has appeared to be in open rebellion, with reports that rebel generals in Jonglei and Unity states are under his command. 

The two American planes were on a mission Saturday to evacuate American citizens, according to a statement from the U.S. military African command, but were fired on as they made their approach. The planes were forced to abort the rescue mission, turned back and flew to Uganda.

"We can confirm that four U.S. service members were injured today from gunfire directed at their aircraft in South Sudan. The aircraft was participating in a mission to evacuate American citizens in Bor,” the statement said.

The aircraft diverted to Kampala, the Ugandan capital, the Associated Press reported. The servicemen, one reported to be in a critical condition, were then flown to Nairobi, Kenya, for treatment.

There were no details on how many Americans were being evacuated, who they were and whether a further rescue effort would be made later.

The U.S. Embassy in Juba, the South Sudan capital, tweeted Saturday that 450 Americans and other foreigners have been evacuated in recent days.

"We will continue to monitor the situation on the ground in order to assess future possibilities for [evacuation] flights from South Sudan," the embassy said.

China National Petrolem Company is also evacuating hundreds of oil workers. 

In recent days, 2,000 youths of the Nuer ethnicity associated with Machar attacked a U.N. compound in Akobo, in Jonglei state, killing two Indian peacekeepers and more than 20 civilians, mainly of Dinka ethnicity, who had taken refuge there.

U.N. peacekeepers are evacuating staff from several bases in Jonglei state as a result, leaving citizens under the protection of the South Sudanese army, according to local media.   

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robyn.dixon@latimes.com