The United Nations Security Council has welcomed the resumption of negotiations between Sudan and South Sudan, but says the two sides must immediately work to implement a peace plan crafted by the African Union.

In a statement Monday, China's ambassador to the U.N. and current Security Council President Li Baodong said the countries have made progress in meeting the requirements of a March Security Council resolution, and he noted a reduction in border violence.

“However the members of the Security Council expressed their strong concern about delays and stressed that important elements of the resolution remained unresolved by both parties.”

He said Council members stressed the urgency of immediately establishing border security mechanisms and said a demilitarized border zone “shall be established.”

The two countries have failed to agree on a map for the demilitarized zone. The African Union says this “critical question” will be part of the agenda when negotiators resume discussions Thursday in Ethiopia.

The Security Council also reiterated “grave concern” about the border states Southern Kordofan and Blue Nile, particularly with what it called the “rapidly worsening humanitarian situation.”

South Sudan split from Sudan last July, but the countries are locked in disputes over border demarcation, oil revenue, and the status of nationals in each other's territory.