"Leaders from the region including the Ugandan president, [South Sudanese President] Salva Kiir, Kenya's president and the Ethiopian prime minister are expected in Arusha for the summit," Abdulrahaman Kinana, the secretary general of Tanzania's ruling Chama Cha Mapinduzi party, told reporters in Juba.
"President Jakaya Kikwete (of Tanzania) arrives in Arusha this evening and is expecting all other leaders to arrive tomorrow," he said.
Kinana said the warring SPLM factions loyal to Kiir and sacked vice president Riek Machar as well as a group of independent SPLM officials are expected to reach an agreement on reunification at the summit.
"Progress has been made very well and hopefully the draft document for the reunification will be signed tomorrow," he said, flanked by former Tanzanian Prime Minister John Samuel Maccello and SPLM spokesperson Akol Paul Kordit.
"We have come to consult with President Kiir on the draft document which we submitted to the heads of states but particularly to the leaders of the three groups, Kiir, Machar and Pagan Amum," Kinana explained.
He revealed that most of the draft document was agreed upon by the three SPLM factions during a two-week intra-party dialogue hosted by Chama cha Mapinduzi in Arusha.
"They discussed the genesis of the crisis on their own and they came up with a solution on their own," said Kinana.
"We passed through Uganda and consulted President Museveni," he noted. "Now we have come to consult President Kiir."
Kinana did not explain much about the document.
"They agreed on political, structural and leadership," he said, adding that the sides have agreed about 95 percent of issues.
"These five, we wish the wisdom of the leaders of the region to guide them to agree on," Kinana hoped.
Kordit, the SPLM spokesperson, confirmed the development.
"Significant process has been made in the talks and general agreement has been made by all parties in the talks," he told reporters. "The SPLM is grateful."
South Sudan has been shaken by violence since December of 2013, when Kiir accused Machar of leading a failed coup attempt against his regime.
Hundreds of thousands of South Sudanese have since been displaced in fighting between the two rivals, close to two million have been uprooted from their homes, and hundreds of thousands now seek shelter in displacement camps across the country.