South Sudanese rebels are in the Ethiopian capital Addis Ababa to thrash out details of a ceasefire to end more than two weeks of crisis in the world’s newest state.

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 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.

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.