JUBA, South Sudan — South Sudanese rebels and government forces fought for control of the capital of oil-rich Upper Nile state, the only region in the world’s newest nation that’s still producing crude two months after violence erupted.

Both sides claimed control of the town of Malakal after fighting started Tuesday in violation of a cease-fire they signed on Jan. 23 in the Ethiopian capital, Addis Ababa.

Fighting that started Dec. 15 has left thousands of people dead and forced at least 860,000 more to flee their homes, according to the United Nations. Tuesday’s clashes probably won’t derail peace talks that resumed last week, U.S. Special Envoy to Sudan and South Sudan Donald Booth said in a interview.

“There’s been continued fighting so I wouldn’t characterize this as one event that could tip things,” he said. “This needs to be brought to a halt to facilitate not only the delivery of humanitarian assistance but to getting on to addressing the root causes of the problem.”

Clashes started in South Sudan after President Salva Kiir accused former Vice President Riek Machar, whom he fired in July, of leading a failed coup. The ensuing violence pitted members of Kiir’s ethnic Dinka community against Machar’s Nuer group.

South Sudanese army spokesman PhilipAguer demanded that regional mediators working to help end the conflict in South Sudan deal with rebels’ violation of the cease-fire.

“They’ve already declared that they don’t respect the cessation of hostilities,” Aguer said. “We’ve been on our positions, defending our positions. We respect the cease-fire.”