South Sudan government troops violated the country’s latest cease-fire just hours after it began at midnight, the armed opposition claimed Saturday, while a government spokesman accused the rebels of attacking instead.

The competing claims indicated a shaky start to the latest attempt at ending the 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.

President Salva Kiir and rival Riek Machar, Mr. Kiir’s former deputy, had agreed on a “permanent” cease-fire earlier this week in neighboring Sudan after their first face-to-face talks in nearly two years.

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.