* Triggered by political rivalry, conflict turns ethnic
* Machar-Kiir meeting would be first since fighting began
* Mediators met rebel leader at base in the bush
By Edmund Blair
NAIROBI, April 28 (Reuters) - International mediators have called on South Sudan's rebel leader to meet his rival President Salva Kiir to prevent an ethnic-fuelled conflict turning into a civil war or genocide, one of the envoys who met Riek Machar last week said on Monday.
EU envoy Alexander Rondos said such a face-to-face between the rivals, which would be the first since fighting erupted in mid-December, was essential to end the "cycle of vengeance" and killings that has left a ceasefire agreed in January in tatters.
"The situation is now so combustible that it has all the ingredients of an all-out civil war in which the consequences could end up being genocidal," Rondos, who is the European Union's envoy to the Horn of Africa region, told Reuters.
Fighting that initially erupted in December because of political rivalry between the president and his sacked deputy Machar has increasingly turned ethnic, largely pitting the president's Dinka people against the rebel leader's Nuer.
Raising the alarm, the United Nations accused rebels of butchering civilians when they seized the oil hub of Bentiu this month, a charge rebels deny. Days later, Dinka residents of Bor town attacked a U.N. base where mostly Nuers were sheltering.
Rondos was in a delegation of African, U.S, British, Norwegian and U.N. envoys, led by the regional African grouping IGAD, that met Machar at his base in the bush in South Sudan.
The group told Machar that he and Kiir should meet to rein in their fighters and end the bloodshed.
"We came away with the impression that he understood the importance of acting responsibly," Rondos said of Machar's response to the proposal. "We await his positive reaction."
Rondos said there were ongoing contacts with Kiir and the government in Juba, and a similar proposal would be put to the president. Rondos said he believed Kiir also understood the "time was right" for a meeting under IGAD auspices.
He did not give details of where or when such a meeting might take place.
Neither Machar nor his officials, who usually have to be contacted by satellite phone, could immediately be reached for comment. South Sudanese presidential or other officials were also not picking up calls seeking comment.
"Both sides are being given a very clear message that they cannot afford anymore to use diversionary tactics, political or otherwise, and that the time has come for the principals (Machar and Kiir) to be seen together and agree on a way to prevent further violence," Rondos said by telephone from Europe.
At the meeting with Machar in Upper Nile state, Rondos said the group also warned the rebel leader that the international community would hold to account all parties involved in fighting in South Sudan, where he said Kiir and Machar were increasingly viewed as leaders of largely ethnic-oriented military forces.
The U.N. Security Council has called for an investigation into the massacres in Bentiu and is considering sanctions on the warring parties. Rights groups say both sides may have committed "war crimes", a status that could lead to prosecution.
The United Nations has approved expanding the UNMISS peacekeeping mission in South Sudan to 12,500 troops from about 7,000 and is also reviewing the existing mandate. UNMISS has sheltered civilians but diplomats say it lacks teeth.
"Given the explosiveness of the situation on the ground, it now becomes an absolute priority to have a revised U.N. mandate which allows for a robust presence of the U.N. to protect civilians and to deter either party from engaging in further violence," Rondos said.
A new round of peace talks between South Sudan's rebels and the government resumed on Monday, although there has been little sign of tangible progress despite months of on-off negotiations.
(Editing by Alison Williams)