|The attacks on Saturday marks an escalations in tensions in the region poised to secede in July [File: EPA]
Rebel fighters have attacked the capital of South Sudan's Upper Nile state, the southern army has said.
The number of casualties is not known, but the attack on Saturday in Malakal, one of the south's three main
settlements, marked an escalation in a wave of clashes between the south's army and rebel fighters.
The fighting has raised fears over the stability of the region in the countdown to its secession - due on July 9.
"There is fighting going on in Malakal. Militia have penetrated the town. They raided at night," Philip Aguer, the southern army spokesman told the Reuters news agency.
Aid workers in Malakal told Reuters they were woken at about 4am (0100 GMT) by the sound of gunfire and shelling.
"We're not that close to the fighting but the buildings here were shaking. I was lying under my bed," said one humanitarian official, speaking on condition of anonymity.
A local resident in Malakal, told Reuters he had seen the bodies of several civilians killed in the clashes.
The southern army said the fighting started in the centre of the town. However, UN official David Gressly said the clashes were concentrated in the north of Malakal, near its airport.
Aguer blamed the attack on fighters he said were aligned with George Athor, a former army officer who rebelled last year saying he had been cheated out of the governorship of neighbouring Jonglei state in elections.
In January, southerners overwhelmingly voted to declare independence in a referendum promised in a 2005 peace deal that ended decades of civil war with the Khartoum-based northern government.
The autonomous southern government has accused the north of backing militias in the south to destabilise the region and keep control of its oil, an allegation dismissed by the government in Khartoum.