The fighting erupted on Monday between the Dinka and the Zande, who form the majority of the population in the Yambio region near the border with the Democratic Republic of Congo, leading to the looting and burning of the UN health agency's compound.

Although the exact circumstances of the clashes were still unclear, preliminary reports indicated that the fighting followed the return of the state governor. Initial scuffles at the airport developed into rioting, with Zandes beating Dinkas.

"Apparently a small incident at the airport sparked the violence in Yambio, which continued throughout the night," David Gressly, the UN deputy resident and humanitarian coordinator for southern Sudan, told IRIN on Wednesday.

Those involved in the conflict fired assault rifles and torched homes. Aid workers, Associated Press news agency reported, saw several dead bodies in the town, but no information on the exact number of casualties was yet available.

The Sudanese head of an agriculture training centre that is affiliated with the NGO Catholic Relief Services was killed as his vehicle was attacked when he tried to enter Yambio town, coming from the nearby town of Nzara.

"It is still unstable, and since we don't have people on the ground in Yambio, the situation is a little unclear - but we are monitoring the situation," Gressly noted.

Due to rising tensions in Tanbura, northwest of Yambio, more aid workers were evacuated.

"We are trying to understand the real dimensions of this. We are trying to understand the underlying tensions in that community," Gressly maintained.

The pastoralist Bor Dinka were displaced during Sudan’s 21-year civil war and settled in the region inhabited by agriculturalist Zande. Tensions have been rising between the two ethnic groups, a source in the region explained, following the decision by Bor Dinka to stay rather than go back to their area of origin in Jonglei State.

"We will support local mediation efforts in order to address some of the issues and foster reconciliation," Gressly noted.