South Sudan President Salva Kiir met regional leaders Wednesday ahead of an expected signing of a peace deal with rebels, amid threats of UN sanctions in the absence of an accord.

Presidential spokesman Ateny Wek Ateny has told AFP that Kiir "will sign the peace agreement", but the government has said it still has "reservations" about some parts of the power-sharing deal.

At least seven ceasefires have already been agreed and then shattered within days — if not hours — in the world's newest country, which broke away from Sudan in 2011.

But the deal, if signed Wednesday, would give rebels the post of first vice president, which means that rebel chief Riek Machar would likely return to the post from which he was sacked in July 2013, six months before the war began.

Machar already signed the deal on August 17, but at that time, Kiir only initialled part of the text. His government then slammed the accord as a "sellout" and said it needed more time for consultations.

The UN Security Council on Tuesday piled pressure on Kiir to also sign, warning it was ready to "act immediately" if he did not.

"We will take immediate action if he does not sign, or if he signs with reservations," said Nigerian Ambassador Joy Ogwu, whose country currently chairs the council.

READ: US circulates draft resolution for UN sanctions on South Sudan

East African leaders, including Ethiopian Prime Minister Hailemariam Desalegn, who has hosted months of talks aimed at ending the 20-month war that has killed tens of thousands of people, arrived for the signing ceremony due later Wednesday.

The conflict has been characterised by ethnic massacres and rape.

First step

Ugandan President Yoweri Museveni — who sent troops to back Kiir during the war, which if the deal is signed, would have to leave within 45 days — is at the talks, along with Kenyan President Uhuru Kenyatta and Sudan's First Vice President Bakri Hassan Saleh.

Rebel leader Machar however is in Ethiopia, as mediators said security provisions were not in place.

Key government concerns include the demilitarisation of the capital Juba, giving greater powers to the rebels, and putting foreigners in charge of a Monitoring and Evaluation Commission — the body that will police the implementation of the peace deal.

The UN envoy to South Sudan, Ellen Margrethe Loej, told the UN Security Council that a peace accord if signed would still be "only a first step" and that many hurdles lie ahead.

The African Union on Wednesday warned any deal had to be not only signed but also implemented, calling on both sides to "commit to genuine reconciliation" and to "put the interests of South Sudan and its people above narrow interests."

South Sudan's civil war erupted in December 2013 when Kiir accused his former deputy Machar of planning a coup, sparking a cycle of retaliatory killings that has split the country along ethnic lines.

Tens of thousands are estimated to have been killed, nearly 70 per cent of the country's population is facing food shortages and some 200,000 terrified civilians are sheltering in UN bases.

The deal before Kiir commits both sides to an immediate end to fighting and the implementation of a "permanent ceasefire" within 72 hours.

Military forces also have 30 days to gather for "separation, assembly and cantonment" — or confinement to barracks, with their weapons secured in storage — with a security review ahead of an eventual reunification of forces.

The peace proposal has been put forward by the regional eight-nation bloc IGAD, the Intergovernmental Authority on Development, as well as the UN, AU, China and the "troika" of Britain, Norway and the United States.