U.N. experts say South Sudan's National Security Service has been operating outside the law and poses "a significant threat" to last September's fragile peace agreement, including by allegedly killing two government critics and arresting others.

The experts monitoring sanctions against South Sudan said "it is highly probable" that opposition member Aggrey Idri and human rights lawyer Dong Samuel Luak were slain Jan. 30 on orders of the director of the National Security Service's Internal Security Bureau, Lt. Gen. Akol Koor Kuc.

Their report to the Security Council circulated Thursday says youth activist Peter Biar Ajak was arrested on his arrival in the capital Juba on July 28, 2018, and has since been detained with only intermittent access to lawyers.

The panel says such actions threaten implementation of the peace agreement.