The South Sudanese army said on Sunday it had repulsed three rebel attacks on its positions near the market town of Bor, which is regarded as a gateway to the capital Juba.
Thousands have been killed and more than 800,000 have fled their homes since fighting began in South Sudan two months ago, triggered by a power struggle between President Salva Kiir and Riek Machar, his former deputy whom he sacked in July.
The warring sides in South Sudan, the world's youngest country after seceding from Sudan in 2011, signed a ceasefire on Jan. 23 but sporadic clashes have continued.
Bor, situated 190 km (120 miles) to the north of Juba by road, has changed hands at least three times since December when fighting broke out between SPLA troops loyal to the government and rebel forces.
"SPLA positions were attacked this morning three times in northern Bor, in the Gadiang (area), but the attacks were repulsed. They attacked from six, seven, eight this morning," SPLA spokesman Philip Aguer has told Reuters via telephone.
Rebel officials were not immediately available for comment.
The raids near Bor followed a rebel assault on Tuesday on Malakal, capital of the main oil-producing Upper Nile region in the north of the deeply impoverished and chaotic country.
Control of Malakal is split between the army and rebels since forces loyal to Machar raided the town and fought government troops. Aguer said that it was just a matter of time before the government retook Malakal from the rebels.
A petroleum ministry official told Reuters on Thursday that national oil output had fallen to about 170,000 barrels per day even before the rebel strike on Malakal, a dip of around a third since the fighting erupted in December.
The fighting in Malakal has jeopardised peace talks in Addis Ababa, already delayed by rebel demands for the release of four remaining detainees and the withdrawal of Ugandan troops from South Sudan.
Kiir's government says Ugandan forces were invited in to help secure key government offices and the Juba airport.