Residents in villages of Jonglei state in South Sudan reported armed militants went on a killing spree and stole cattle from their community.
The U.N. peacekeeping mission for South Sudan reports 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.
The statement said it took note of recent negotiations in Ethiopia but called for more resolve from both sides of the conflict.
"Swift resolution of these outstanding issues will advance security and prosperity for citizens of both countries" the statement read. "We further call on the parties to refrain from any further destabilizing actions or inflammatory language that might jeopardize the relations between both states, and in that context note with concern the recent and dangerous escalation of military action along the Sudan-South Sudan border."