But the Security Council decided not to impose a threatened arms embargo.
Tens of thousands of people have died since the civil war started in December 2013.
It began following a political row between President Kiir and his former deputy Riek Machar, who was accused of mounting a coup attempt.
Several peace efforts have failed, but it was thought that August`s regionally-backed deal that created a power-sharing government would finally work.But the UN experts say that both sides are "persistently failing to implement a permanent ceasefire" and that they are "actively expanding their stockpiles of arms and ammunition".
They also say that there are credible reports that killings and sexual violence continue in Unity state, in the north of the country, with more than 50 rapes being recorded in October.
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