At least 62 rebels and militiamen were killed in a clash with soldiers in South Sudan's oil-rich Upper Nile and Unity states, army spokesman Lul Ruai Koang says.
The rebel supporters of former deputy president Riek Machar attacked an army base near the town of Malakal on Thursday and Friday, according to Koang.
At least 35 others were injured, he said.
However, Colonel Gatjieth Gatluak, a spokesman for the rebels speaking to dpa by telephone from neighbouring Sudan, accused the army of initiating the clashes.
South Sudan sank into chaos when a power struggle between President Salva Kiir and Machar, who is currently in South Africa receiving medical treatment, escalated into a military conflict in December 2013.
Tens of thousands have been killed and about 2.5 million displaced.
The formation of a unity government in April did not stop the fighting, and Machar later fled the country.
