The competing claims indicated a shaky start to the latest attempt at ending a devastating five-year civil war that has killed tens of thousands and created Africa's largest refugee crisis since the 1994 Rwandan genocide. Millions are near famine and aid delivery is often blocked in one of the world's most dangerous countries for humanitarian workers, AP reported.
President Salva Kiir and rival Riek Machar, Kiir's former deputy, had agreed on the "permanent" cease-fire earlier in the week in neighboring Sudan after their first face-to-face talks in nearly two years. They then ordered their supporters to observe it.
Opposition spokesman Lam Paul Gabriel said government forces and Sudanese rebel militias launched a "heavy joint attack" in Mboro, Wau County in the northwest around 7 a.m. Saturday, arriving in armored personnel carriers, trucks and Land Cruisers.
"The fight is still ongoing as I write," Gabriel said, calling on the UN peacekeeping mission and cease-fire monitors to investigate. The opposition reserved the right to self-defense, he added.
South Sudan government spokesman Ateny Wek Ateny said that the opposition attacked instead.
"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," Ateny said.
The previous cease-fire in December was violated within hours, prompting a new push by the international community to threaten UN and regional sanctions against those blocking the path to peace.
This time, Kiir and Machar had faced a possible UN arms embargo and sanctions if fighting didn't stop and a political deal wasn't reached by Saturday.