* Government loses control of capital of oil-producing state
* Ethnic division fuels conflict in world's newest country
* UN trying to move in aircraft from neighbouring regions (Recasts with rebels holding Bentiu, mediator, other details)
By Carl Odera and Ben Makori
JUBA, Dec 22 (Reuters) - South Sudan's government said on Sunday rebels had seized the capital of a key oil-producing region and fears grew of all-out ethnic civil war in the world's newest country.
The U.N. announced it was trying to rush more peacekeeping forces to landlocked, impoverished South Sudan as foreign powers urged both sides to stop fighting, fearing for the stability of an already fragile region of Africa.
The South Sudan government said on its Twitter account it was no longer in control of Bentiu, the capital of Unity State.
"Bentiu is not currently in our hands. It is in the hands of a commander who has declared support for Machar," it said.
Information Minister Michael Makuei said on Saturday an army divisional commander in Unity State, John Koang, had defected and joined rebel leader and former Vice President Riek Machar, who had named him the governor of the state.
But the government in Juba said it was still in control of the oilfields crucial to the economy.
U.N. Secretary General Ban Ki-moon told a news conference in Manila the U.N. planned to send resources from other peacekeeping missions in the region to South Sudan.
"We are now actively trying to transfer our assets from other peacekeeping missions like MONUSCO (in the Democratic Republic of Congo) ... and some other areas," he said.
"And we are also seeking support from other key countries who can provide the necessary assets."
Clashes between rival groups of soldiers in the capital Juba a week ago have spread across the country, which won its independence from Sudan in 2011 after decades of war.
President Salva Kiir, from South Sudan's Dinka ethnic group, has accused Machar, a Nuer whom he dismissed in July, of trying to launch a coup. The two men have long been political rivals.
Machar dismissed the charge but has since said he is commanding troops fighting the government.
MACHAR "ESCAPES BY BOAT"
Government soldiers had come across Machar with a group of fighters in Bor, Foreign Minister Barnaba Marial Benjamin said, without giving details.
"Riek managed to escape, used his boat along the Nile and ended up in his village of Ado and went into Bentiu (the administrative capital of Unity) ... the night before, he attacked government institutions," Benjamin added.
On Friday, mediators from other African states met President Kiir in Juba for what they called "productive" talks. His government said it was willing to hold talks with any rebel group.
Kenyan Lieutenant-General Lazarus Sumbeiywo said on Sunday mediators had not yet made contact with Machar to hear his side of the story.
"I don't think it is feasible at the moment under the circumstances ... and so we will find another way of getting to Riek Machar. Not through Juba," Sumbeiywo told Reuters.
The army acknowledged losing Bor in the northern Jonglei State on Wednesday, and the United Nations said oil workers had taken refuge in its bases in neighbouring Unity.
Reuters television footage showed the government sending more troops on Saturday to Bor - the scene of an ethnic massacre of Dinka in 1991 by Nuer fighters loyal to Machar.
Benjamin said Machar had not seized oilfields in Unity.
"Of course there is a threat. But ... he is not occupying the oilfields. The oil has been running."
Speaking in Khartoum, South Sudan's Ambassador Mayen Dut Wol also said oil was flowing normally. South Sudan's output of 245,000 barrels per day supplies almost all government revenues and hard currency to buy food and other vital imports.
The United Nations says hundreds of people have been killed in the conflict and around 62,000 people have been forced to flee their homes in five of South Sudan's 10 states. Around 42,000 of them were seeking refuge at U.N. bases, it added.
U.N. BASES LOOTED
"Looting of humanitarian compounds has been reported in Jonglei (Akobo and Bor) and Unity. Several U.N. and NGO compounds in Bor town have reportedly been completely looted, including vehicles stolen," the U.N. Office for the Coordination of Humanitarian Affairs said in a report.
A spokesman for U.N. peacekeepers said they were bringing in more aircraft from their logistics base in Entebbe in Uganda to South Sudan.
A diplomatic source at the U.N. in New York said elements of the U.N. intervention brigade in eastern Congo could help out in South Sudan, but would only reinforce security at U.N. bases and not try to confront armed groups.
The source said the U.N. had asked countries to help it get real-time satellite images of South Sudan and there was a possibility of using unmanned surveillance drones, currently deployed in eastern Congo but that would take time.
The U.N. peacekeeping mission in South Sudan said on Sunday it was relocating non-essential staff and planned to reinforce its military presence in Bor and Pariang to protect civilians.
The spokesman said about 100 civilian staff of the South Sudan peacekeeping mission were being relocated on Sunday, and 60 staff from other U.N. agencies left on Saturday.
Three U.S. aircraft came under fire from unidentified forces on Saturday while trying to evacuate Americans from the conflict. The U.S. military said four of its members were wounded in the attacks.
A U.S. defence official said the U.S. military was looking at other options to evacuate the remaining Americans from Bor.
The U.N. mission in South Sudan said one of four U.N. helicopters sent to Youai, in Jonglei state, had come under small-arms fire on Friday. No crew or passengers were harmed. (Additional reporting by George Obulutsa in Nairobi, Philippa Croome in Kampala, Lou Charbonneau in New York, Khaled Abdel Aziz in Khartoum and Missy Ryan in Washington; Editing by Andrew Roche)