South Sudan must arrest killers to stop conflict: HRW

South Sudan must honour promises to investigate, arrest and prosecute those responsible for an explosion of bloody ethnic violence in war-wracked Jonglei state, Human Rights Watch said Friday.

"To stem this horrific cycle of violence, the organisers have to be held to account," said Daniel Bekele, HRW's Africa director, adding UN and African bodies should help for "speed and credibility’s sake."

"The government has repeatedly promised to investigate the attacks and hold those responsible to account, but it has not made any apparent progress in investigations or arrests," the New York-based rights group added.

Grossly impoverished South Sudan, the world's newest nation which declared independence last July, is reeling from a wave of bloody ethnic violence and rebel attacks.

Last month a marauding column of up to 8,000 armed youths from the Lou Nuer people marched on the remote town of Pibor, home to the rival Murle, whom they blame for cattle raiding and have vowed to exterminate.

Scores of people are being treated for machete and gunshot wounds at clinics, and over 140,000 people have been affected by the attacks and reprisal raids.

UN peacekeepers mandated to protect civilians have been unable to reliably assess how many people were killed during the brutal attacks last month.

"Murle leaders reported that more than 3,000 had been killed, while UN monitors have been able to confirm just a fraction of that figure and have not released an estimate of total casualties," HRW added.

Eyewitness testimonies gathered by the aid agency Doctors Without Borders (MSF - Medecins Sans Frontieres) include reports of babies beaten against trees and women forced to watch their children have their throats slit.

HRW said a witness who visited an attack site days later had seen 12 dead bodies, including three women apparently "raped with blunt objects."

"This goes far beyond traditional cattle-rustling," Bekele said.

"The conflict is far more vicious, involving the deliberate targeting of villagers, including women and children, for abuse and has taken on dangerous ethnic and political overtones."

UN peacekeepers and the government knew days in advance the militia column was advancing, but South Sudan's army failed to deploy in time.

That left forces in the area "too greatly outnumbered to intervene" HRW said, with government officials and UN peacekeepers telling civilians to flee for their lives ahead of the marauding force.