OLD FANGAK, Sudan, April 22 (Reuters) - Southern Sudanese villagers say government-backed militiamen are attacking them in an extortion campaign that is undermining faith in a peace accord meant to end civil war in Africa's largest country.

Residents of Old Fangak, an isolated settlement in Upper Nile State, and aid workers say militia groups which are not signatories to the accord sealed in January pose a big threat to the oil-exporting country's bid to end 21 years of conflict.

"There is a government militia group led by a man called Gabriel Tanginya who has been attacking our villages, and demanding illegal taxes from our people," said Isaac Gatkho Kuol, an official in the village on the Zerafi River.

"People are always expecting Tanginya to attack this area. ...People do not believe there is peace, they do not read newspapers, they have no radio, they see no evidence of the peace so how do they know? We have to show them," Kuol said.

Under security arrangements set out under the deal between the government and the rebel Sudan Peoples Liberation Movement/Army (SPLM/A), militia groups must join either the SPLA or government forces within 12 months from the signing.

But humanitarian organizations say militia groups like the one led by Tanginya are yet to join either one and continue to harass and rob the poor inhabitants of the area.

"Militia activities are still a problem," said Klaus Stieglitz of German rights group Sign of Hope. "This taxation continues to place yet another hardship on a people struggling to rebuild their lives after suffering war and displacement."

Khartoum denies militias are robbing civilians. "There have been clashes between tribal groups in the area but these have no connection to the government," an official army source said.

Tanginya's groups is one of several pro-Khartoum armed bands which did not accept invitations to a meeting in Nairobi this week where southern churches, political parties and voluntary groups pledged unconditional support for the peace accord.

Their militias' involvement in the peace process is seen as vital because they also fought in the war and some fear they may be used by Khartoum to sabotage a southern authority due to be created, and likely dominated, by the SPLM/A in coming years.

TAXED A LOT

The north-south war between the Arabic-speaking Islamist government and southern animist and Christian rebels seeking greater autonomy, was complicated by oil, ethnicity and ideology.

There are no roads, no cars and no bicycles in this desolate region of Sudan. People have to walk, sometimes for days, through the sweltering heat and dust, risking snake and scorpion bites to get to the nearest big town, Malakal, 80 miles away.

Locals also use the Zerafi to transport goods by canoe or motorboat. It is along its banks that militias erect checkpoints where they tax villagers boating to government-held Malakal.

"If you are going to buy sugar, or even new clothes for the children you will be taxed a lot of money. The soldiers have no other way of making money so when you buy goods they tax you," said Rebbeca Alek, a 40-year-old mother of seven.

"The only thing they do not tax is the clothes on your back," she says, swatting swarms of flies settling on her face.

The Civilian Protection Monitoring Unit (CPMT), a U.S.-led body charged with monitoring attacks against southern civilians, has documented several recent militia attacks.