* South Sudan in turmoil since sacking of vice president
* Rebels want civil society groups based abroad involved
* Officials warn of famine if conflict drags on too long
By Carl Odera and Aaron Maasho
JUBA/ADDIS ABABA, June 23 (Reuters) - Rebels have boycotted a new round of peace talks to end South Sudan's conflict because of a dispute about who should attend, those involved in the protracted negotiations said on Monday.
South Sudan has been in political turmoil since President Salva Kiir sacked his deputy Riek Machar last year, triggering a conflict that has reopened deep ethnic tensions in the world's youngest country which only won independence from Sudan in 2011.
Kiir comes from the dominant Dinka ethnic group, while Machar, now rebel leader, is from another major tribe, the Nuer.
The new round of talks in the Ethiopian capital Addis Ababa was to have brought in new players such as civil society organisations, faith-based groups and others, along with government and rebel negotiators, to try to secure a broad deal.
There has been little to show for months of negotiations. Two ceasefire deals were swiftly violated, with each side blaming the other. A new delay in talks threatens to extend a crisis that aid agencies say could drive the nation to famine.
The regional African IGAD grouping, which is mediating, has threatened both sides with sanctions unless they halt fighting.
In a statement, IGAD said the rebel delegation had boycotted the expanded negotiations, breaking a deal reached in May between Kiir and Machar to renew the ceasefire and support broad-based talks.
"FAIR PROCESS" DEMANDED
Machar's spokesman James Gatdet Dak said rebels stayed away because civil society groups from abroad were not included, while those he said were aligned to the government were there.
"So we are protesting against this, we want the other civil society organisations from abroad to be included so that the process is fair and transparent," he told Reuters by telephone.
A diplomat said the rebels had also argued that civil society and other groups should only play a "consultative" role in the talks between the government and the Machar camp.
The government said IGAD had picked the civil society groups. The minister in charge of the president's office, Awan Guol Riak, said groups based abroad could be included later.
The fighting in South Sudan erupted in mid-December after months of political tensions sparked by the sacking of Machar. Thousands have died in the conflict and more than 1.3 million people have been driven from their homes.
IGAD said its mediators would use the adjournment in the negotiations to conduct broad consultations with South Sudanese parties, as well as international actors such as the U.N. Security Council.
"If it is a delay tactic on anyone's part, it will be an extremely costly one to them," said Alex Rondos, the European Union's special envoy to the Horn of Africa.
He told Reuters that action by either side to stall the process could lead to moves "to create a sanctions regime".
A senior U.N. official said this month South Sudan could only avoid a famine in coming months if a shaky ceasefire held and displaced people were able to return home to plant crops.
(Writing by Edmund Blair; Editing by Gareth Jones)