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A ceasefire in South Sudan has broken down after a few hours, with fighting resuming between rival forces in the war-torn African country on Saturday.

Government troops are said to have violated the ceasefire, opening fire on rebel fighters just hours after the midnight ceasefire came into effect, the opposition told AP.

Government forces and Sudanese rebel militias launched a "heavy joint attack" in Mboro, Wau County using armoured vehicles and land cruisers in the 7 am assault, opposition spokesman Lam Paul Gabriel said.

"The fight is still ongoing as I write," Gabriel said.

He called on the UN peacekeeping mission and cease-fire monitors to investigate the incident.

He said that the opposition reserved the right to self-defense.

"This is disappointing that even when their president and commander-in-chief Salva Kiir declares a cease-fire, the regime's forces still violate it," Gabriel told the news agency.

"There is the possibility Salva Kiir is not in control of his forces or he doesn't want peace to come."

A government spokesperson blamed the rebels for breaking the ceasefire.

"They have a loose leadership; they're not being controlled by anyone. The people of South Sudan should be given a chance to lead a peaceful life, and the army is observing the order of the president. It's very sad," South Sudan government spokesman Ateny Wek Ateny told AP.

A ceasefire in December also ended after a few hours with the UN threatening tough sanctions if the truce didn't work this time, including neighbouring countries who have a hand in the conflict.

South Sudan's has killed tens of thousands of people, with President Salva Kiir's government forces facing fighters loyal to rival Riek Machar, the president's former deputy.

A peace deal agreed in 2015 broke down in 2016 with Machar having to flee the country.