JUBA South Sudanese rebels have attacked several villages in the oil-rich east of the new African country and are marching in large numbers on further communities, military and local officials said on Thursday.
The violence threatens government plans to explore a huge oilfield with the help of France's Total and other foreign oil firms in the state of Jonglei, where thousands have died in tribal clashes in recent years.
South Sudan won independence from Sudan last year but its government has struggled to assert control over vast territories awash with weapons after decades of civil war with Khartoum ending in 2005.
Army spokesman Philip Aguer said rebels with the David Yau Yau militia in South Sudan had attacked villages in the county of Twic East on Wednesday. The army was pursuing them, he added, without giving details.
Several lawmakers from Jonglei said Yau Yau was moving a 6,000-strong force in three columns north towards the counties of Akobo East, Akobo West and Twic East.
"We are telling the national government, the state government, the international community that there is something going on in Jonglei state," said parliamentarian Kutin Bayak Gil. "If it continues the way it is now, there should be another disaster that will take place."
James Sigin Banak, another MP from Jonglei, said Yau Yau had recruited members of the Murle tribe which wanted to avenge attacks by the rival Lou Nuer tribe almost a year ago.
Nearly 900 people died when about 7,000 armed youths of the Lou Nuer tribe attacked Murle villages in the Pibor area at the end of last year, according to the United Nations.
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 programme.
"I have just returned from Uror and Pajut where I have seen a lot of fear among the disarmed communities who told me that an attack by the Murle Yau Yau-led movement is imminent. They could strike at any time," said Reverend Canon John Jok, another lawmaker from Jonglei.
Army spokesman Aguer said he did not know the strength of Yau Yau's forces but added: "Yau Yau exploits people who want arms. Since he is giving out free arms, there could be many who have joined him."
South Sudan has accused Sudan of supporting Yau Yau and other militias in the new republic, charges Khartoum denies.
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