JUBA South Sudanese rebels have seized a military base and town after clashing with the army in the east, in an escalation of violence that has already uprooted thousands of people and hampered plans to explore for oil.
The rebels, led by David Yau Yau and known as the South Sudan Democratic Army (SSDA), say they want to end corruption and the one-party system led by the Sudan Liberation People's Movement.
In March, the army launched an offensive against Yau Yau in the eastern state of Jonglei, the country's largest, where the government wants to search for oil with the help of French firm Total.
The recent fighting has uprooted tens of thousands of people, according to the United Nations.
In an emailed statement, Peter Konyi Kubrin, an SSDA spokesman, said rebels found more than 50 dead bodies in the town after the army fled. The figure could not be independently verified.
Army spokesman Philip Aguer said there had been fighting and that the army withdrew from their base in Boma to the top of a mountain, known as Upper Boma, around two hours' walk from the main town.
"It's (Boma) divided with the army on the top of the mountain and the rebels at the bottom. It's just a matter of time before we chase them away," Aguer said, but did not give any casualty figures.
Since winning independence from Sudan in 2011, South Sudan has been struggling to impose its authority across vast swathes of territory teeming with weapons after decades of civil war with Khartoum.
Yau Yau, a former theology student, first rebelled in 2010 after failing to win a seat in state parliament. He accepted an amnesty in 2011 only to take up arms again a year later.
Despite a recent thaw in relations between Sudan and South Sudan, Aguer said the rebels were receiving assistance from Sudan's National Intelligence and Security Services in the form of airdrops of weapons, ammunition and supplies.
"We are convinced beyond reasonable doubt that Khartoum has been supplying them up to now," Aguer said.
The Geneva-based Small Arms Survey, which tracks weapon supplies, said in a recent report much of the rebels' equipment comes from Khartoum. Sudan has routinely denied the charges.
(Editing by Ulf Laessing and Raissa Kasolowsky)