JUBA South Sudanese rebels and government soldiers clashed in the northern town of Nasir on Sunday, adding to fears that a shaky ceasefire agreement signed in May could totally collapse.
Rebel spokesman Lul Ruai Koang said troops loyal to Riek Machar, South Sudan's former vice president, launched an assault on the government-held town and "at dawn liberated Nasir".
South Sudan army spokesman Philip Aguer confirmed there was fighting but said the rebels were not in full control of Nasir, a small town 30 km from the Ethiopian border in the oil-rich Upper Nile State.
"The rebels have been shelling Nasir since last night and this morning at 6am they launched a ground attack," Aguer told Reuters. "There is still fighting in Nasir but the rebels don't control Nasir."
Nasir has changed hands several times since fighting broke out in the capital Juba between soldiers loyal to President Salva Kiir and troops backing Machar in mid-December, quickly spreading across the country.
The violence, triggered by a power struggle between the two men, has often followed ethnic faultlines, pitting Kiir's Dinka against Machar's Nuer.
European Union and the United States have sanctioned commanders from both sides for violating the first ceasefire that was signed in January but swiftly crumbled.
Fighting in Nasir follows last week's skirmishes between rebels and government troops in Unity State, an oil-rich region where the violence has forced complete shutdown in production.
The attack on Nasir is the biggest threat so far to the shaky ceasefire signed by Machar and Kiir on May 9. Both sides have repeatedly accused each other of violations.
"This morning's attack by the rebels of Riek Machar might put the ceasefire to an end," Aguer added.
Thousands of civilians have been killed and more than a million forced to flee their homes since December, prompting U.N. warnings of a famine in some parts of the country, which declared independence from Sudan in 2011.
Oil output, South Sudan's economic lifeline, has been cut by a third to about 165,000 barrels per day since fighting began.
(Additional reporting and writing by Drazen Jorgic; Editing by Raissa Kasolowsky)