South Sudan gained independence in July as part of peace agreement reached with Washington's help in 2005. The deal ended a brutal civil war though disputes over oil and border skirmishes are threatening the fragile peace.
Both sides this week reached a six-point cease-fire agreement to be monitored by an African Union observer mission. The deal wasn't signed, however, because South Sudanese negotiators rejected claims they were backing rebel groups fighting Sudanese forces in the border states of South Kordafan and Blue Nile, the independent Sudan Tribune reports.
Washington announced the authorization of up to $26 million in emergency aid for those caught up in the conflict in the border states.
"The United States continues to call upon Sudan and South Sudan to exercise maximum restraint and to reach a negotiated settlement to the outstanding issues between them," Tommy Vietor, a spokesman for the U.S. National Security Council, said in a statement.
U.S. President Barack Obama spoke Monday with South Sudanese President Salva Kiir to express concern about the border clashes.