South Sudanese rebels have launched an offensive to retake a key town near the border with Ethiopia in what the United Nations said was a clear violation of a truce agreement.
"This attack represents the most serious resumption of hostilities," since President Salva Kiir and his former deputy, rebel leader Riek Machar, met in May and recommitted to a January ceasefire, the UN mission said in a statement on Sunday.
The fighters loyal to Machar on Sunday struck Nasir, their former headquarters located 500km north of Juba and close to the border with Ethiopia. The headquarters was retaken by government forces in May.
UNMISS, the UN mission, laid the blame for the truce violation squarely with Machar's forces. "The attack is a clear violation of the Cessation of Hostilities Agreement," it said.
But a spokesman for Machar's forces, Lul Kuang, defended their moves as an act of "self-defence" after what he described as several government attempts to arrest their military commander.
"The fall of Nasir now paves the way for military resources to be refocused on Poloich Oil Fields, Maban and Malakal," Kuang said in a statement, referring to the main remaining oil field still active.
South Sudanese army spokesman Philip Aguer denied Nasir had fallen, describing the fighting as "still ongoing".
Aguer said army forces had staged nine offensives on Nasir on Sunday.
Only days earlier, mediators had pressed the rival sides to resume peace talks being held in Ethiopia, or face increased sanctions.
Those talks, moderated by East African leaders, are designed to lead to a transitional government and ceasefire but have yielded little progress so far.
Fighting had abated since May in part due to heavy rains and poor roads that hampered troop and equipment movements.
Meanwhile, the violence in South Sudan last December has left thousands dead and displaced 1.5 million people, and aid agencies warn of a looming famine.