South Sudan's army says it has lost contact with its troops in the key oil hub of Malakal following a major offensive by rebels.
The rebels have said they now control the northeastern town after launching an assault on Tuesday, throwing into doubt a ceasefire agreement signed in Ethiopia last month.
"I have no contact with the command in Malakal," army spokesman Philip Aguer told AFP on Wednesday, as the independent radio Tamazuj said a top government general had been seriously wounded in an attack reportedly involving tens of thousands of rebel fighters.
"There was the sound of heavy shelling this morning and then sporadic shooting. It seems the opposition control a part of the town and the airport, but the government soldiers are still fighting," an aid source said.
"We are in no position to confirm who is in control of Malakal town because our people have not been able to get into town," said Joe Contreras, spokesman for the UN mission in South Sudan (UNMISS).
Rebel military spokesman Lul Ruai Koang said opposition fighters were "chasing" government soldiers into the bush, and insisted that it was government soldiers who attacked first and violated the truce.
The United Nations said that 10 people had been killed on Tuesday in "inter-communal clashes" within a peacekeeping base in Malakal.
Rebel fighters have also been reported to be patrolling outside the UN camp, taunting people from rival ethnic groups.
The unrest in South Sudan has left thousands dead and displaced 873,000 people, including tens of thousands who have crammed into UNMISS bases in fear of ethnic attacks by either President Salva Kiir's Dinka tribe or rebel leader Riek Machar's Nuer tribe.
