A general in South Sudan's army has told the BBC the government in the north is arming militias accused of being behind recent ethnic violence.
Maj-Gen Kuol Deim Kuol said his SPLA army was trying to disarm the local population but was being hampered by the continuing supply of weapons.
At least 185 Lou Nuer people were killed in Jonglei state when reportedly attacked by Murle fighters on Sunday.
Several hundred people have died in such clashes this year.
The UN says this is more than in Sudan's Darfur conflict.
Violence over land and cattle in South Sudan is exacerbated by a ready supply of firearms following the 22-year civil war with the north, which ended in 2005.
"There must be a force somewhere, a force that keeps arming these militias, a force that keeps sending ammunition to the militias," Maj-Gen Kuol told the BBC's Network Africa programme.
"There is not another force in this way that can keep arming and sending ammunition to the local population apart from the Sudanese army and the [northern governing] National Congress Party," he said.
Northerner officials have previously denied similar accusations, claiming southern politicians want to shift the blame for their failure to establish peace and restore security since the end of the war.
Officials in Jonglei said members of the Lou Nuer community had gone fishing south of Akobo town amid a severe food shortage when they were attacked.
Eleven SPLA soldiers, who were guarding their camp, were among those killed.
An aid worker who has worked in the area told the BBC's World Today programme that the clashes in Jonglei have escalated.
"Many people have been displaced into Akobo town, some have gone south into Pibor - everyone is worried because this hasn't happened on this scale before," she said.
"Since 2005 there have been some disarmaments but there are still very many people in South Sudan in these two tribes who are still armed," she added.
Analysts say the violence comes at a critical time for Sudan, as tensions grow in the north-south unity government.
Elections are due in April 2010, the first chance to vote for many in decades.
After that, a 2011 independence referendum is due for the south, which many believe will see Africa's biggest nation split fully in two.