* Fighting near the flashpoint town of Bor
* Mediator fears fighting could scupper talks
* White House says will deny support to those responsible (Adds comments by South Sudan defence minister on new fighting, detail on delegation)
By Carl Odera and Aaron Maasho
JUBA/ADDIS ABABA, Jan 1 (Reuters) - South Sudanese rebels arrived in the Ethiopian capital Addis Ababa on Wednesday to thrash out details of a ceasefire to end more than two weeks of bloodletting that has threatened to plunge the world's newest state into civil war.
The U.N. envoy to South Sudan, Hilde Johnson, said government negotiators were also on their way, with both sides under mounting pressure from regional and Western powers to reach a deal.
The White House has said it would deny support - vital in a country the size of France that still has hardly any infrastructure more than two years after secession - to any group that seizes power by force.
Both sides have agreed in principle to a ceasefire but neither has indicated when the fighting would stop and mediators are concerned that fighting around the flashpoint town of Bor will scupper the talks even before they begin.
South Sudan's defence minister said government forces were battling rebel fighters 11 miles (18 km) south of Bor, the capital of Jonglei state, which has untapped oil reserves and was the site of an ethnic massacre in 1991.
"There will have to be a fight because they want to defeat the government forces," Defence Minister Kuol Manyang Juuk told Reuters from the capital Juba, 190 km south of Bor by road.
Rebels loyal to former Vice President Riek Machar seized control of Bor on Tuesday after tens of thousands of civilians had fled.
The Addis Ababa talks will focus on finding ways to roll out and monitor the ceasefire, the East African IGAD bloc that is mediating the talks said, to end the fighting that has killed at least 1,000, unsettled oil markets and raised fears of the conflict spilling over in a fragile region.
"We don't want to expose the people of South Sudan to a senseless war," South Sudan's Foreign Minister Barnaba Marial Benjamin said on a government Twitter feed on Wednesday.
South Sudan's neighbours, Washington and the United Nations played a central role in negotiations that ended decades of war with Sudan to the north and led to the secession of South Sudan in 2011, and have been scrambling to stem the latest violence.
South Sudan holds the third-largest oil reserves in sub-Saharan Africa after Angola and Nigeria, according to BP, but remains one of the continent's least developed countries.
WHITE HOUSE PRESSURE
President Salva Kiir has accused his long-term political rival Machar, who he sacked in July, of starting the fighting in a bid to seize power.
Machar has denied the charge, but he has taken to the bush and has acknowledged leading soldiers battling the government.
Clashes between soldiers erupted on Dec. 15 in Juba. The violence quickly spread to oil-producing areas, dividing the country along the ethnic lines of Machar's Nuer group and Kiir's Dinkas.
Machar ally Rebecca Nyandeng, a senior figure within the ruling Sudan People's Liberation Movement (SPLM), which her late husband John Garang founded, told Reuters Machar's delegation would be led by former Unity state governor Taban Deng Gai.
A Reuters witness saw Gai, along with two other members of the rebel delegation arrive at Addis Ababa's Sheraton hotel.
Gai was a long-serving governor until he was sacked by Kiir in July - the same month Kiir fired Machar as vice president.
Many of the senior SPLM figures who were critical of Kiir and accused him of being dictatorial and monopolising power within the party were arrested when fighting broke out, with the authorities labelling them as coup plotters.
"We need to reform, democratise the (SPLM) party," said Nyandeng, who has been critical of Kiir in the past. "So I think they will talk on those issues."
The White House upped the pressure for the talks on Tuesday.
"We will hold leaders responsible for the conduct of their forces and work to ensure accountability for atrocities and war crimes," spokeswoman Caitlin Hayden said.
The United Nations Mission in South Sudan said ethnic-based atrocities, often carried out against civilians by uniformed men, have taken place across the country.
Fighting across the country has displaced at least 180,000 people, according to the United Nations.
The clashes have revived memories of the factionalism in the 1990s within the SPLM, the now ruling group that fought Sudan's army in the civil war. Machar led a splinter faction at the time and Nuer fighters loyal to him massacred Dinkas in Bor. (Additional reporting by Drazen Jorgic and Richard Lough in Nairobi, Jeff Mason in Honolulu and Kumerra Gemechu in Addis Ababa; Writing by Drazen Jorgic; Editing by Richard Lough and Alison Williams)