The talks that opened on February 5 in Addis Ababa are aimed at finding a settlement to one of Africa's worst conflicts.

"We are hoping that there will be some form of agreement signed tomorrow," UN envoy David Shearer told reporters at UN headquarters in New York on Thursday.

"It might not go as far as we had hoped, but it might provide the platform for ongoing discussions."

Talks in Ethiopia have focussed on governance and security in South Sudan.

The African Union and the UN Security Council have threatened to impose sanctions on those who are blocking peace efforts in South Sudan, which this year entered its fifth year of war.

The fighting has killed tens of thousands of people, uprooted nearly four million -- roughly a third of the population -- and raised the risk of famine.

Shearer said a ceasefire deal reached in December was holding despite violations, stressing that "the overall level of violence has come down."

"I'm encouraged by where we are right now," said the envoy, who also heads the UN peacekeeping mission in South Sudan, which has deployed 14,000 troops and police.

"But it still very much hangs in the balance."

South Sudan descended into war in December 2013 when President Salva Kiir accused his former deputy Riek Machar of plotting a coup.

One million South Sudanese have crossed borders to become refugees as they flee the violence.