* UN finds grave in Bentiu thought to hold 75 Dinka soldiers
* Says two more mass graves reported in Juba
* EU says South Sudan risks "spiralling into disaster" (Adds President Kiir's comment about government troops taking over Bor)
By Carl Odera and Aaron Maasho
JUBA, Dec 24 (Reuters) - A mass grave believed to contain the bodies of 75 ethnic Dinka soldiers has been found in South Sudan, the United Nations said on Tuesday, fuelling fears of ethnic bloodletting in the world's newest state.
President Salva Kiir said on Tuesday government troops had retaken control of Jonglei state capital Bor, a key town which last week fell to rebels loyal to former Vice-President Riek Machar, who was sacked by Kiir in July.
U.N. human rights officers discovered the grave in the rebel-held city of Bentiu, capital of Unity state.
"We have discovered a mass grave in Bentiu, in Unity State, and there are reportedly at least two other mass graves in Juba," U.N. High Commissioner for Human Rights Navi Pillay said in a statement in Geneva.
The conflict in South Sudan erupted in the capital Juba on Dec. 15 but has spread to oil-producing regions and beyond, rapidly dividing the two-year-old nation on ethnic lines.
Much of the fighting has involved Dinka and Nuer factions of the Sudan People's Liberation Army, with militias and marauding youths also reported to be attacking opposing ethnic groups.
Western powers and east African states, keen to prevent more chaos in a fragile region, have tried to mediate between Kiir, a Dinka, and rebel leader Machar, a Nuer.
Both men say the conflict is political, not tribal, but 40,000 people have sought refuge in the bases of U.N. peacekeepers amid widespread fears of ethnic killings.
"My cousin and nephew were both caught and executed. How can I leave this place?" asked Gatjang, a 29-year-old Nuer, at a U.N. compound in Juba crammed with about 12,000 fearful civilians. "Even here. What if they sneak inside and attack us?"
Kiir on Tuesday said government troops were back in Bor after losing the town on Sunday. "Forces loyal to the government have taken Bor and (are) now clearing whatever forces that are remaining there," he told journalists at his office in Juba.
While both men have said they are open to dialogue, Machar said on Monday he would only negotiate if his detained political allies are released, a demand the government swiftly rejected.
"I am extremely concerned that South Sudan risks spiralling into a disaster for both its own people and the region," the European Union's foreign policy chief Catherine Ashton said on Tuesday. "Such a situation can, and must, be avoided." (Additional reporting by Stephanie Nebehay in Geneva; writing by Drazen Jorgic; Editing by Alistair Lyon and David Evans)