By Hereward Holland

JUBA (Reuters) - South Sudan's army has killed 13 people suspected of belonging to a rebel militia in the troubled eastern state of Jonglei, a local official said on Tuesday, the latest violence threatening government plans to explore a huge oilfield.

South Sudan won independence from Sudan last year but its government has struggled to assert control over vast territories where many people have weapons after decades of civil war with Khartoum ending in 2005.

In Jonglei a cycle of revenge killings between the Murle and Lou Nuer tribes and a heavy-handed government disarmament campaign have eroded hopes that independence would bring peace.

The government plans to explore a vast oilfield in Jonglei with the help of France's Total and other foreign oil firms.

The army, composed of poorly trained former militiamen and guerrillas, shot dead 13 people on December 4 near the town of Gumuruk in Pibor county, local commissioner Joseph Konyi said.

"According to the army they were ambushed by the rebels. But locals said (those killed) were civilians. We don't know which is true," Konyi told Reuters.

The army was not available to comment despite repeated attempts to reach it. The government has played down accusations by rights groups that its security forces committed abuses against civilians during a campaign to disarm civilians and end cattle raiding.

Pibor's former human rights commissioner, Peter Gazulu, said the dead were civilians and were not members of a local insurgency run by David Yau Yau, a former theology student.

"My brother in law was killed there. Also a young boy related to my wife was killed," Gazulu told Reuters.

Human rights groups accuse the army of fuelling dissent by committing abuses including rape and torture when it launched a disarmament of the Murle and Lou Nuer early this year aimed at ending bloodshed between the two groups.

The army denies the charges. Many Murle resisted disarmament and fled into the bush where they joined the Yau Yau, residents in Jonglei say.

A shortwave radio station with links to the Yau Yau rebellion said the group was fighting the government in reaction to abuses committed during the disarmament program.

Nearly 900 people died when about 7,000 armed youths of the Lou Nuer attacked Murle villages in the Pibor area at the end of last year, according to the United Nations.

(Reporting by Hereward Holland; Editing by Ulf Laessing and Mark Heinrich)