* Government disarmament drive criticised for abuses
* Juba says Khartoum supplying Murle militia revolt
* Conflict could hamper development of big oil reserves
* Jonglei is already a tinderbox of tribal tensions
A heavy-handed government disarmament campaign to halt tribal clashes in South Sudan's swampy eastern grasslands has triggered a small armed revolt against the rulers of the world's newest state, threatening planned oil exploration in the area.
The budding insurgency in Jonglei state led by Murle militia chief David Yau Yau, a former theology student, may not number more than a few dozen fighters.
But there are fears it could escalate by feeding on local grievances against South Sudan's army.
The leaders of former civil war foes Sudan and South Sudan struck a border security deal this week that should be enough to restart suspended oil exports from the South, which became independent from Sudan last year.
But with both sides still deeply mistrustful after decades of enmity, serious doubts remain over whether the uneasy neighbours can share their oil wealth in peace.
Anti-government unrest in Jonglei stems from a muscular disarmament campaign by the South Sudanese military, the Sudan People's Liberation Army (SPLA), to prevent a repeat of clashes in January between the Murle and Lou Nuer tribes which killed several hundred people.
New York-based Human Rights Watch said in August it received credible reports that elements of the SPLA had engaged in killings, rape, beatings and torture during the disarmament campaign, dubbed "Operation Restore Peace".
The United Nations peacekeeping mission in South Sudan (UNMISS) also reported similar abuses, saying most of the victims were women and in some cases children.
"This is a recipe for recruitment. The disarmament has created an environment ripe for creating a non-state armed group. It's utterly predictable," said one aid worker in the country who asked not to be named.
South Sudan's army played down the reports of abuses, but said they had already dismissed 30 soldiers.
Authorities in Juba last week accused Sudan of airdropping weapons and ammunition to Yau Yau's rebels in Jonglei state, which is the site of a vast unexplored oil concession that the government recently split into three.
"There are hawks within the (ruling party) in Khartoum. There are those who are bent on thinking that they can only resolve the issues with the South through war and bringing a lot of instability in this country, supporting various militia groups," said South Sudan's government spokesman, Information Minister Barnaba Marial Benjamin.
"Their intention is to scare off investment, that is basically it," he added.
Sudan's government and military routinely deny Juba's accusations that they are backing insurgencies.
OIL AT STAKE
At its independence in July last year, South Sudan inherited the bulk of the oil output produced by previously united Sudan.
Juba shut down its 350,000 barrels per day at the start of the year, alleging Khartoum was 'stealing' its oil by diverting it from pipelines through Sudan. The deal struck by Sudanese President Omar Hassan al-Bashir and his southern counterpart, Salva Kiir, in Addis Ababa is expected to restart oil exports.
Key to South Sudan's future oil development plans, which include a proposed alternative pipeline that would take southern crude through Kenya or Ethiopia rather than Sudan, is a massive oil concession that covers much of Jonglei state.
This month the government in Juba decided to break up the huge Block B largely held by Total into three blocks, giving one to the French company and the others to two more foreign firms.
Total had stopped operations in the block in 1985 after the resumption of Sudan's long civil war, which finally ended with a 2005 peace deal paving the way for the South's secession.
Fresh conflict in Jonglei state, where thousands of Murle and Lou Nuer have been killed in bouts of ethnic feuding stretching back decades - although these days automatic rifles have replaced spears and knives - would unnerve prospective investors and embarrass the government.
In August, Yau Yau's fighters killed at least 24 SPLA soldiers in an ambush. Another 74 South Sudanese troops are still missing and may be dead, the army says.
Local leaders blame abuses committed by SPLA soldiers during the disarmament campaign for pushing local recruits into the insurgency, especially members of Yau Yau's Murle tribe who feel they have been unfairly targeted.
"If the (army) did not beat civilians, Yau Yau would not have come back because he knew he would not get support," Ismail Konyi, a member of parliament and a senior Murle leader, said.
"Now I think Yau Yau will recruit these youths easily."
The Murle tribe see themselves as victims of long-standing persecution and marginalisation by the government in Juba.
REBEL ALLIANCE FORMING?
In an email statement, another rebel group, the South Sudan Liberation Army (SSLA), which attacked several towns and planted land mines in Unity state in November last year, says it is on the brink of forming an alliance between anti-government forces, including Yau Yau's fighters.
"All the rebel groups have formed an alliance which would be announced in a month. Major General David Yau Yau is a commander of revolutionary forces in Jonglei. We are all one," read the email sent from an account bearing the SSLA name.
The SSLA, which South Sudan also accuses Khartoum of backing, says that Murle militia chief Yau Yau has even courted the rival Lou Nuer militia as a potential ally in his revolt against the government in Juba.
The Lou Nuer's "White Army" led by purported prophet Dak Kueth was responsible for the massacre of 600 Murle in December and January and was also targeted in the army disarmament drive.
Government spokesman Benjamin, himself a Lou Nuer member of parliament, rejected the assertion that the White Army could join Yau Yau.
Yau Yau, thought to have limited military experience, first rebelled in May 2010 after standing as an independent candidate in the state's parliamentary election for the Gumuruk-Boma constituency. He lost to the SPLM candidate by a big margin.
During his first revolt, he gained support amongst the youth because he was seen as a champion of Murle interests. But he also lost backing when he accepted a South Sudan government amnesty in June 2011, allegedly in exchange for a house, cars and cash, according to Murle involved in the negotiations.
He defected to Khartoum in April while supposedly being treated in a Kenyan hospital, and later went back to Jonglei with 19 men, arriving in July, Murle leader Konyi said.
According to a radio station called Radio Yau Yau, which the Juba government believes is broadcasting from Khartoum, his rebels are fighting in reaction to abuses committed during the disarmament programme, especially the rape of Murle women.
Konyi presents himself and other Murle elders as key mediators to defuse the tense situation.
They have asked the South Sudanese army to end disarmament, prosecute those responsible for the abuses, change the SPLA commanders in Pibor and help civilians return home.
But the underlying sense of marginalisation felt by some in the newborn African state will take some time to fade away.
"We Murle, in reality, don't belong to Jonglei. When everything is divided we are not given anything - food, water, jobs, roads," Konyi said. "We have some tribes ... who don't even know if there is a government operating."