South Sudan president Salva Kiir pictured at the Intergovernmental Authority on Development (IGAD) 29th Extraordinary Summit in Addis Ababa on January 29, 2015

South Sudan's President Salva Kiir has finally agreed to sign a peace deal and power-sharing accord to end a 20-month civil war, his spokesman said Tuesday.

Presidential spokesman Ateny Wek Ateny told AFP that the presidents of Kenya, Uganda and Sudan plus Ethiopia's prime minister "will converge on Juba tomorrow morning for a one day summit, and the President of the Republic of South Sudan will sign the peace agreement."

The spokesman said, however, that the government was still unhappy with the accord, drawn up by the regional bloc IGAD.

"The government has some reservations... even if the President will sign," Ateny said.

South Sudan's rebel leader Riek Machar, a former vice president, signed the deal last Monday, in line with a deadline to do so. Both sides in the conflict have been facing the threat on international sanctions if they refuse to sign.

But Kiir only initialled part of the text, and his government slammed the accord as a "sellout" -- saying it needed more time for consultations.

Key issues of disagreement include details of a power-sharing proposal between the government and rebels, which could see Machar return as vice-president.

Ateny also said the government was unhappy over calls to demilitarise the capital Juba, hand over greater powers to the rebels in the oil-rich Upper Nile region, and see foreigners in charge of a Monitoring and Evaluation Commission -- the body that will police the implementation of the peace deal.

Sources in IGAD also confirmed plans for the deal to be signed in Juba on Wednesday, with Kenyan President Uhuru Kenyatta, Ugandan President Yoweri Museveni, Sudanese President Omar al-Bashir, Ethiopian Prime Minister Hailemariam Desalegn and chief mediator Seyoum Mesfin due to attend.

An IGAD official said rebel leader Machar would not be there because security provisions were not yet in place.

- Plans for 'Truth, Reconciliation and Healing' -

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

Marked by widespread atrocities on both sides, the war has been characterised by ethnic massacres and rape.

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.

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

The 72-page accord commits both sides to implementing a "permanent ceasefire" within 72 hours after signing.

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.

All foreign forces in the war, including Ugandan troops backing Kiir, must leave within 45 days, while foreign militia forces, including rebels from neighbouring Sudan's Darfur and Nuba mountain regions, must also be disarmed and sent home.

No troops are allowed closer than 25 kilometres (15 miles) to the capital Juba, with only presidential guards, police and guard forces protecting infrastructure can remain in the city.

The deal gives rebels the post of "First Vice President", alongside the current vice-president. That means Machar would likely return to the post he was sacked from in July 2013, six months before the war began.

Signatories also take responsibility for the war, "apologising unconditionally" for the tens of thousands killed.

A Commission for Truth, Reconciliation and Healing will be set up to investigate "all aspects of human rights violations", with a "Hybrid Court" -- set up in collaboration with the African Union -- to try crimes including possible genocide and crimes against humanity.