Amnesty International called Tuesday on the South Sudanese authorities to investigate "shocking human rights violations" on civilians by members of the country's security forces.
"South Sudan should take immediate action to end human rights violations including torture, shootings and sexual violence by security forces carrying out a civilian disarmament campaign in Jonglei State," the London-based rights group said in a statement.
The disarmament programme by South Sudan's security forces began in the volatile eastern state following a wave of ethnic violence that erupted in late December.
The attacks left nearly 900 people dead, according to UN figures, after an estimated 8,000-strong militia force from the Lou Nuer people rampaged through Jonglei's Pibor county, massacring members of the rival Murle group, abducting women and children, razing villages and stealing cattle.
The response of the army -- made up of former rebel fighters who went on to form the South's official army when the country became independent in 2011 -- has been heavily criticised for its brutal treatment of civilians.
Far from bringing security to the region, South Sudan's army and police force "have committed shocking human rights violations and the authorities are doing very little to stop the abuse", said Audrey Gaughran, Amnesty's Africa director.
In August, Human Rights Watch reported "soldiers shooting at civilians, and ill-treating them by beatings, tying them up with rope, and submerging their heads in water to extract information about the location of weapons" in the disarmament scheme.
The United Nations also said in August it was concerned about an increase in "serious human rights violations allegedly committed by some undisciplined elements within the South Sudanese Army" and voiced fears that the actions threatened to derail peace efforts there.
Jonglei was one of the areas hardest hit in Sudan's 1983-2005 north-south civil war, which ended in a peace deal that paved the way for the South's full independence.
But the new nation is awash with guns, while heavily armed communities that were once pitted against each other during Khartoum's rule remain rivals.