Secretary of state urges leaders gathered in Kenya to improve transparency.
Clashes then broke out between about 70 soldiers and the fighters, said to be from the Lou Nuer ethnic group.
Koul said that the livestock that was taken was returned to their owners – from the Dinka tribe – and that the situation was calm.
The fighting is believed to have escalated from seasonal cattle-raiding to revenge killings.
Violence in south Sudan has led to the deaths of more than 1,000 people this year.
However, this year more people have died in the annual cattle raiding and women and children have been targeted for the first time since a 2005-peace deal that ended a 22-year long north-south civil war.
More than 250,000 people have been displaced in inter-tribal fighting across the south since January and aid workers are warning of its worsening effect on an already dismal humanitarian situation in the region.
An excess of cattle on the southern market has exacerbated problems for some tribes people, who receive a reduced amount of grain in exchange.
The Dinka and Lou Nuer tribes are major southern pastoralist groups with a long history of inter-tribal cattle raiding.