At least 24 South Sudanese soldiers have been killed in clashes with a militia group that the southern government accuses of subverting a disarmament campaign, a southern military official said on Monday.

South Sudan army spokesman Col. Philip Aguer said that an additional 12 soldiers were wounded and 17 are still missing from the Aug. 22 ambush of 200 South Sudanese troops who had been sent to quell alleged rebel activity in Jonglei State, the scene of a disarmament program that Human Rights Watch criticizes as too violent.

The killing of South Sudanese troops has been condemned by the U.N. mission in South Sudan, which said in a statement that the attack was "deliberately intended to undermine progress made in improving the security environment and in forging inter-communal reconciliation in Jonglei State."

Aguer said the rebels are led by an elusive militant named Yau Yau and that they are backed by South Sudan's northern neighbor Sudan in a clandestine effort to keep Jonglei State lawless and ungovernable. Aguer said the rebels are allied with the cattle-keeping Murle tribe, whose men are resisting the government's disarmament efforts. The soldiers were ambushed at a place called Fhodo in Pibor County, he said, describing the rebels simply as a "Khartoum-supported militia." Sudan has consistently denied such allegations.

Relations between the two Sudans have been especially tense since April, when the southern military forcibly took a disputed oil well in an armed conflict that threatened to escalate into full-blown war between the neighbors.

South Sudan became an independent state last year, but it has outstanding border and oil-related issues with Sudan. The neighbors are currently engaged in on-and-off negotiations mediated by the African Union.