* U.S. military aircraft shot at while on evacuation flight
* Mediators seek meetings with President's rivals
* Army commander defects, joins Machar
U.S. aircraft came under fire on Saturday on a mission to evacuate Americans from spiralling conflict in South Sudan and four U.S. military service members were wounded.
Nearly a week of fighting threatens to drag the world's newest country into an ethnic civil war just two years after it won independence from Sudan with strong support from successive U.S. administrations.
The U.S. aircraft came under fire while approaching the evacuation site, the military's Africa Command said in a statement.
"The aircraft diverted to an airfield outside the country and aborted the mission," it said.
Hundreds of people have been killed in the fighting that pits loyalists of President Salva Kiir, of the Dinka ethnic group, against those of his former vice president Riek Machar, a Nuer who was sacked in July and is accused by the government of trying to seize power.
Fighting that spread from the capital, Juba, has now reached vital oilfields and the government said a senior army commander had defected to Machar in the oil-producing Unity State.
After meetings with African mediators on Friday, Kiir's government said on its Twitter feed that it was willing to hold talks with any rebel group. The United States is also sending an envoy to help with talks.
South Sudan's foreign minister, Barnaba Marial Benjamin, told Reuters the African mediators had now been given the go-ahead to meet with Kiir's rivals, including Machar and his allies. They were due to make contact on Saturday.
United Nations staff say hundreds of people have been killed across the country the size of France this week and 35,000 civilians are sheltering at their bases.
Information Minister Michael Makuei told Reuters that an army divisional commander in Unity State, John Koang, had defected and joined Machar, who had named him the governor of the state.
The United Nations said on Friday at least 11 people from the ethnic Dinka group had been killed during an attack by thousands of armed youths from another ethnic group on a U.N. peacekeeping base in Jonglei state. Two Indian peacekeepers died.