South Sudanese rebels have launched an offensive to retake a key town near the border with Ethiopia in what the UN says is 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 struck Nasir, their former headquarters, located 500km north of Juba and close to Ethiopia, which was retaken by government forces in May.
UNMISS, the UN mission, laid the blame squarely for the renewed fighting 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 in activity.
South Sudan, the world's youngest nation, marked only the third anniversary of its independence this month under the shroud of a war which has enflamed ethnic hatred and brought its people to the brink of famine.
The conflict has raged since mid-December when presidential guards loyal to Kiir clashed with troops supporting Machar, who fled to the bush and rallied a huge rebel army.
An estimated 1.5 million people have been displaced and thousands killed, including civilians massacred indiscriminately in hospitals and churches and dumped in mass graves.
