African mediators try to avert civil war in South Sudan JUBA (Reuters) - African mediators said they held "productive" talks on Friday with South Sudan's President Salva Kiir, trying to prevent an almost week-long conflict plunging the world's newest nation into an ethnic civil war. In a sign of the nervousness among South Sudan's neighbors, Ugandan soldiers flew in to help evacuate their citizens. Two anonymous military sources said they would also help secure the capital, which lies about 75 km (50 miles) from Uganda's border. Kiir, a member of the Dinka ethnic group, has accused his former vice president Riek Machar, a Nuer who was sacked in July, of attempting to seize power by force. Fighting that began on Sunday in the capital Juba has swiftly spread, and U.N. staff have reported hundreds killed. Machar told French radio he was ready to "negotiate his departure from power" and said the army could force Kiir out unless he quit. U.S. Secretary of State John Kerry said he spoke with Kiir on Thursday and would send his envoy for Sudan and South Sudan, Ambassador Donald Booth, to help facilitate talks. 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. The violence must stop, the dialogue must intensify," Kerry said in a statement. As fighting has moved out of the capital, it is increasingly driven by ethnic loyalties as much as political rivalries. The U.N. 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. The United Nations had earlier said at least 20 people were killed, and South Sudan's government said earlier 54 Dinka had been killed in the incident. The United Nations mission in South Sudan is still trying to verify the exact number of dead. Deputy U.N. peacekeeping chief Edmond Mulet told the U.N. Security Council on Friday 35,000 civilians were sheltering at U.N. bases across the country. Fighting has spread to oilfields vital to the impoverished new state's economy and dependent on foreign workers. Soldiers from the rival factions clashed at a barracks near the town of Bentiu, capital of the oil-producing Unity State. Some 200 oil workers sought refuge in a U.N. base on Thursday. It's difficult to find plane providers to fly to some of these remote air strips. The situation on the ground is very fluid and we can't be absolutely confident about exact rebel locations and which airfields they may be controlling," said one security analyst, who did not wish to be identified. The mediation team visiting Juba included ministers from Kenya, Uganda, Djibouti and Somali, and African Union and United Nations representatives. It was the first peace initiative since clashes erupted. "We had a very productive meeting with his Excellency President Salva Kiir and we will continue consultations," Ethiopian Foreign Minister Tedros Adhanom, who is leading the African delegation, told reporters before returning to talks. The fighting worries neighboring states, which fear new instability in a volatile region. It threatens the halting steps towards the creation of a functioning state in a country which declared independence from Sudan in 2011 after decades of war. "Some troops from (Uganda's) Special Forces Command - I can estimate in hundreds - left for Juba yesterday," said a source in the Command, a unit led by President Yoweri Museveni's son. "They will mainly be involved in securing the capital," he said. Some had gone by plane and others would travel by road. They're not going to participate in the skirmishes between Kiir and Machar. President Barack Obama said the United States had sent 45 military personnel to protect the embassy and U.S. employees. "This force will remain in South Sudan until the security situation becomes such that it is no longer needed," he said in a letter to Congress. Obama's national security adviser, Susan Rice, urged leaders to avoid the kind of violence seen in Rwanda and Darfur. "If individuals or groups seek to take or hold power through force, mass violence, or intimidation, the United States will have no choice but to withdraw our traditional, robust support" for South Sudan, she said. "Killing will only lead to deprivation and isolation for the people of South Sudan," Rice said in an audio message she recorded for the nation. A Kenyan transporters' body said trade through landlocked South Sudan had ground to a halt, with hundreds of trucks stuck at border crossings with Uganda and Kenya. "We have a military coup in our hands which is causing a lot of instability in the country and is being played up in certain areas as if it is a racial ethnic war, which is not the case," Foreign Minister Barnaba Marial Benjamin told Reuters. "We don't want to encourage what happened in Rwanda," he said, referring to the 1994 genocide there. Clashes in Bor town, where Nuer in 1991 massacred Dinka, have fuelled the fears of an all-out ethnic war. Nuer commander and Machar ally, Peter Gadet, now controls Bor, officials said. Political tensions between the two politicians had been mounting since Kiir, facing growing public frustration about the slow pace of development, sacked Machar. Speaking to France's RFI radio, Machar said that unless Kiir quit office "I think the people will depose him, in particular, influential people in the army." Before the fighting erupted, Kiir accused his rivals of reviving the kind of splits in the ranks of the ruling SPLM party that led to bloodshed in 1991. Additional reporting by Drazen Jorgic in Nairobi, Aizu Chen in Singapore, Maggie Fick in Cairo, Elias Biryabarema in Kampala, Lou Charbonneau in New York and Joseph Akwiri in Mombasa, Roberta Rampton and Arshad Mohammed in Washington; Writing by Edmund Blair; Editing by Andrew Roche and Jonathan Oatis