At least six civilians have died in fighting in South Sudan in the past two days, rebels said, and both rebels and the government accused one another of planning a return to full-blown conflict after a lull in the rainy season.
Civil war has killed more than 10,000 people in the world's newest state, reopened deep fault lines among ethnic groups, caused more than one million to flee and driven the country of 11 million closer to famine.
Diplomats and analysts have warned there could be a spike in fighting as the dry season approaches, after relative peace in the rainy season.
Violence erupted in December 2013 in South Sudan after months of political tension between President Salva Kiir and his sacked deputy and rival, Riek Machar. The conflict pits Kiir's Dinka ethnic group against Machar's Nuer.
South Sudan military spokesman Philip Aguer said government forces were expecting another round of attacks in oil-producing Unity State, and Upper Nile State, which experienced fighting in recent days.
He said fighters allied to Machar had staged attacks on Saturday near Bentiu, Unity state's capital.
"The SPLA forces in those areas are monitoring preparation of Riek Machar forces to wage attacks possible within this coming week," Aguer told Reuters.
Rebel military spokesman Lul Ruai Koang on Saturday accused government troops, police and pro-government militia of a series of attacks in the past two days in Unity and Upper Nile States in which at least six civilians were killed. The government denied that it had caused any deaths.
Peace initiatives by the regional group of nations, the Intergovernmental Authority for Development (IGAD), have so far failed to end the conflict.
IGAD said last week it was also concerned with the resumption of fighting in the two states.
"IGAD Monitoring and Verification Teams have been instructed to investigate these latest incidents and determine responsibility for the violations," it said in a statement on January 1.
Fighting has cut oil production by about a third to an average 160,000 barrels per day (bpd) since fighting broke out, from 245,000 bpd just before the violence erupted.