Khartoum-backed militia in Sudan's South Kordofan state have killed 17 civilians, rebels said on Saturday.
They accused the group of ethnic South Sudanese of ambushing a civilian lorry on Friday at Abu Nuwara, about 80 kilometres (50 miles) from the border with South Sudan's Upper Nile state.
"They clashed with the civilians there and there's a lot of casualties," said Arnu Ngutulu Lodi, spokesman for the Sudan People's Liberation Movement-North (SPLM-N) which has been fighting government forces in South Kordofan since June 2011.
He said the incident occurred in a government-controlled area and blamed a militia linked to Lam Akol's Sudan People?s Liberation Movement-Democratic Change (SPLM-DC).
"This is ridiculous," Akol told AFP from Khartoum. "We don't have a militia."
SPLM-DC is South Sudan's main opposition party, a breakaway group from the Sudan People's Liberation Movement which has ruled the South since independence in 2011 after a 22-year civil war.
He said the rebels in South Kordofan are "just parroting what their masters in Juba are saying."
Khartoum accuses South Sudan of supporting the SPLM-North, and this has been a major obstacle for the failure of Sudan and South Sudan to implement key security and economic agreements signed in September.