The U.N. Security Council is warning of "costs or consequences" for South Sudan's government and opposition groups if they undermine upcoming efforts to achieve a cease-fire and implement a 2015 peace agreement.

The council says in a statement approved by all 15 members Thursday that "no party should set preconditions to participation."

Council members strongly backed the forum organized by an eight-nation East African regional group to revitalize peace efforts. It is expected to begin in the coming week.

The world's newest nation plunged into ethnic violence in December 2013 and clashes continue despite the peace agreement.

As a first priority, the Security Council calls on all parties to end hostilities "as a sign of commitment" to the peace process.