South Sudan's plan to hold elections next year risks "deepening and extending" an already devastating civil war, the United Nations warned.

Haile Menkerios, the UN secretary-general's special envoy to the African Union, expressed concern about the elections planned for July 2018 during a joint meeting of the visiting UN Security Council and the African Union's Peace and Security Council.

South Sudan's election can only be held in a stable environment where "people are not displaced by violence and hunger and in which they are able to express their political views free from intimidation," the envoy said on Friday.

President Salva Kiir seeks an election that will be the first vote on his leadership since South Sudan won independence in 2011 from Sudan.

Presidential elections set for 2015 were delayed by the civil war that began in late 2013.

Tens of thousands of people have been killed in the conflict and two million people have fled the country, while the UN estimates that six million people, or half the population, are severely food insecure.

Multiple attempts at peace deals have failed, and the UN and aid groups have said South Sudan has become the most dangerous country in the world for humanitarian workers.

The United States this week imposed sanctions on two senior South Sudan government members, a former official and three companies accused of undermining peace, security and stability in the East African nation.

South Sudan's leadership "has failed to fulfil the hope of this young nation and need to be told that there's no alternative to inclusive and genuine dialogue to resolve the differences," the chair of the African Union's Peace and Security Council, Smail Chergui, told Friday's meeting.