There are fears that the clashes between the forces belonging to the two sides in the unity government could threaten the fragile peace deal and election due to be held next year.
The fighting broke out on Thursday between the military and the armed wing of the main opposition SPLM-IO party in various areas in the northern oil-producing state of Upper Nile.
The army spokesman accused the opposition forces of attacking its positions in Longichuk and Malual Gathoth. The SPLM-IO says it was attacked.
On Monday, the SPLM-IO, led by deputy President Riek Machar, withdrew from the country’s peace monitoring body because of what it said were unprovoked attacks.
Nicholas Hysom, head of the UN’s peacekeeping mission in South Sudan (Unmiss), said that decision was deeply worrying.
Quote Message: Less than 12 months of the transitional period remain, it is crucial that all parties continue their best efforts to sustain the ceasefire and work towards implementing all outstanding benchmarks so that free and fair elections can take place.
Quote Message: There is no military solution to the conflict in South Sudan. I encourage all political parties to put aside their differences in the larger interest of peace, progress and prosperity.”
A civil war broke out in South Sudan 2013 when President Salva Kiir and Mr Machar fell out, leading to a deadly conflict that forced about four million people from their homes.