Rebel forces have attacked positions held by government troops, killing 14 of them and wounding 42 others, military spokesman Col. Philip Aguer told The Associated Press.
Government troops have killed 38 rebels and have captured two others along with 50 rocket-propelled grenades, he said.
The clashes, which started Monday and continued yesterday, took place in Leer and Koch, he said. The death toll of 52 was for fighting in Koch and not Leer, which government troops had to abandon under intense fire from the rebels. Aguer gave no civilian death toll from the fighting.
"The rebels are planning to attack Malakal and Nasir (in Upper Nile). Right now as I am talking to you they are mobilising their forces around Malakal and we expect them to attack anytime," he said.
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It was not immediately possible to get a comment from the rebels.
South Sudan President Salva Kiir and his rival, former Deputy President Riek Machar, signed a peace deal in August and both sides blame each other for violating the latest cease-fire.
The country has been hit by a civil war since December 2013 when Kiir, an ethnic Dinka, fired his deputy Machar, an ethnic Nuer. The fighting has often been marked by ethnic violence.