Teams with the U.N. Mission in South Sudan during the weekend said they expected clashes in Jonglei state between members of the Lou Neur and Murle communities.
Hilde Johnson, U.N. special envoy to South Sudan, made an impassioned plea for civilian protection in the region.
"Time is now running out and the government needs to redouble its efforts to prevent a tragedy and avert large-scale violence," she said in a statement. "All South Sudanese people must now put peace and stability in their new and independent country above any other concerns and interests."
Residents in villages of Jonglei state in South Sudan reported armed militants went on a killing spree and stole cattle from their community earlier this month.
The U.N. peacekeeping mission for South Sudan reported that around 45 people were killed and many others fled the area following the attacks.
UNMISS notes Jonglei state, the center of civil conflict in Sudan in the 1980s, has experienced the worst of the ethnic violence since South Sudan became an independent country in July.
A statement from the British, Norwegian and U.S. governments called on Sudan and South Sudan to settle differences over border disputes and the sharing of oil revenue.