US ambassador Susan Rice is warning that Sudan's refusal to accept a map demarcating the border with South Sudan could lead to the resumption of "outright conflict".
Rice told reporters after a closed Security Council meeting Thursday that the United States was "deeply concerned" about the lack of urgency from both countries, but especially Sudan, in implementing an African Union roadmap aimed at averting a new war.
The Security Council endorsed the plan in May, adding specific timelines.
Rice said Sudan's failure to agree to the plan is preventing the establishment of a demilitarized zone border zone between the two countries.
The AU is mediating talks to try to resolve outstanding issues by a September 22 deadline set by the Security Council.