(CNN) -- More than 65 people have been killed in two days of clashes between rebel groups and soldiers in Southern Sudan's Upper Nile state, an army spokesman said.
Philip Aguer, spokesman for the Sudan People's Liberation Army (SPLA) told CNN.com that forces loyal to a militia leader named Oliny attacked the village of Uach west of Malakal in Upper Nile state.
"55 members of the militia and seven SPLA soldiers were killed, and we believe a number of civilians also died, but we are still getting information."
Oliny, Aguer said, is a former member of Southern Sudanese politician Lam Akol's militia. "But we don't know if he is still associated with him."
Aguer also said that he believes that Oliny was receiving military support from the government in northern Sudan.
"They have received new weapons. We suspect they all acted in coordination with Khartoum ... I think things are going to continue escalating," he said.
However, Sudan's dominant National Congress Party (NCP) in the north denied having any involvement.
Rabie Abdelati, an NCP party official, said on Monday: "If we really wanted to go back to war, we would not have signed the Comprehensive Peace Agreement (the 2005 accord) or accepted the referendum.
"We are hoping for a strong south after secession. If the south is not stable the north will not be stable," he said.
Akol, a seasoned Southern Sudanese politician and a former member of the SPLA, broke from the SPLA in 2009 and created a new party, the Sudanese Peoples Liberation Movement-Democratic Change (SPLM-DC).
Akol rejected any association with Oliny's militia.
"He was never a part of the SPLM-DC," he told CNN.com.
"He is a Shiluk youth who with others were defending their land from Dinka who tried to take their land," he told CNN.com.
The fighting took place in areas dominated by the Shiluk tribe, which Akol belongs to. The SPLA on the other hand is dominated by members of the Dinka tribe.
The fighting comes days after more than 40 people had been killed in clashes between soldiers and another rebel militia, George Athor, in Southern Sudan,
Rebel leader George Athor has been saying his forces have killed more than 100 people, most of them soldiers, since a fresh round of fighting started last week.
But Philip Aguer, a spokesman for Southern Sudan's military, said Athor's claims are exaggerated.
Athor took up arms in 2010 when he was not elected governor of Southern Sudan's Jonglei state.
The fighting comes just four months before the region is due to become independent.
The south is expected to secede on July 9 after southerners overwhelmingly voted to declare independence from the north in a January referendum -- a vote promised in a 2005 peace accord that ended decades of civil war between north and south.