South Sudan in 2011 became an independent country but since then disputes over oil, ethnic conflicts and skirmishes along the shared border have threatened the peace.
Both sides have expressed support for settling outstanding differences by creating demilitarized zones along the border and resuming oil production.
Victoria Nuland, a spokeswoman for the U.S. State Department, said in a statement that Washington was "disappointed" that neither side can live up to its own agreements, however.
"Lack of resolution on this issue prevents normalized relations between Sudan and South Sudan and compounds the current human rights and humanitarian emergency," she said. "The African Union and United Nations must stand firm and demand that the parties urgently uphold their commitments to avert this threat to regional peace and stability."
The independent Sudan Tribune reports that South Sudanese President Salva Kiir and Sudanese President Omar al-Bashir are to meet this week on the sidelines of the annual African Union summit.