Victoria Nuland, a spokeswoman for the U.S. State Department, said the White House condemned an attack Sunday by Sudanese forces on the South Sudanese state of Unity.
"We recognize the right of South Sudan to self-defense and urge South Sudan to exercise restraint in its reaction to Sudan's attack in Unity state and to refrain from disproportionate actions which would only further inflame the hostilities between the parties," she said in a statement.
Tensions between both sides escalated after South Sudan captured Heglig, an oil town near ill-defined borders of the countries. South Sudan last week pulled its forces from the region, though Nuland said more was expected.
The Council of the European Union said Monday it was "deeply concerned" by escalating conflict between both countries.
"The use of force will not resolve any of the outstanding issues between the two countries," an EU statement read.
South Sudan became an independent nation under the terms of a 2005 agreement with Sudan. Border tensions, ethnic conflicts and disputes over oil are threatening the fragile peace, however.