The council made the request Thursday in a statement applauding the start of the latest round of negotiations in Ethiopia and calling for the talks to be fully inclusive.
Thousands of people are believed to have died in two months of conflict between troops of President Salva Kiir and supporters of former Vice President Riek Machar.
About 870,000 people have fled their homes, the Security Council said.
Seven political leaders were released in late January, and the council said four still detained should be freed so they could join political and civic groups in "participating in the political dialogue and a reinvigorated constitutional process."
The council condemned "targeted violence" against civilians and ethnic groups. It demanded aid groups be allowed to travel safely, expressing concern that deteriorating humanitarian conditions and the approaching rainy season could lead to famine.
The United Nations urged both sides to cooperate fully with U.N.-sponsored troops in the country and the Intergovernmental Authority on Development, a regional trade bloc, and warned of "serious consequences" if the conflict was allowed to spread to other regions.