At least 16 people died when rebels attacked South Sudanese troops in troubled Jonglei state, breaking a ceasefire signed in January, a Southern Army spokesman said on Thursday.
"We are still waiting for full details of the casualties," said Philip Aguer, spokesman for the South's Sudan People's Liberation Army (SPLA), adding that he feared the death toll would rise.
"The rebels launched their attacks on Wednesday at 1.00 pm (1000 GMT) killing four of our soldiers, but the army fought back and 12 of the attackers were killed," Aguer told AFP.
Aguer said new clashes broke out on Thursday morning in Fangak town, in the North of Jonglei, the South's most populous state.
"The men of Athor attacked again this morning but they were repelled. The situation is under SPLA control, but we don't know if they will attack."
The clashes shattered a "permanent ceasefire" signed between supporters of rebel leader George Athor and the SPLA in January, just days before a landmark referendum on independence for the South. Athor did not himself attend the signing ceremony in the regional capital Juba.
"The violation of the deal is something that is very disappointing," Aguer said.
"Indeed, we were surprised by the attack, because the SPLA were busy transporting food to the sites of Assembly, where the men of Athor are due to gather under the terms of the agreement."
Athor is a renegade Southern general who launched a rebellion after claiming he was cheated in an election for the governorship of Jonglei last April.
Officials have charged he has used the ceasefire period to recruit more fighters.
"We repeat our appeal to General George Athor to see sense and return to peace," Aguer said.
The rebels attacked two sites in Fangak county, including the main Fangak town. Aguer said the fighting lasted for about three hours until the SPLA took control of the situation.
"The attackers also planted anti-vehicle mines which blew up two SPLA trucks," the spokesman said, adding that three soldiers were wounded in the blasts.
The attacks come just days after the formal confirmation of the results of the January 9-15 independence referendum, which showed almost 99 per cent of southerners voted to secede and split Africa's largest country in two.
Previously the southern army had accused Athor and his men of acting on behalf of Khartoum in a bid to destabilise the South, a charge northern officials have denied.
Analysts have said that maintaining security in the fledgling southern nation will be a major challenge.
Clashes in oil-producing Upper Nile state earlier in February killed 54 people and wounded 85, according to United Nations estimates, when southern former militiamen inside the Northern Army rebelled against orders to surrender their heavy weapons.
On Wednesday, a gunman shot dead Southern minister Jimmy Lemi Milla in his office, an attack reported to be motivated by a personal dispute.