Machar, who led a decade-long breakaway movement among his Nuer people before rejoining the Sudan People's Liberation Movement/Army (SPLM/A) in 2002, was on Friday appointed by Sudan's First Vice President and President of the government of southern Sudan, Salva Kiir Mayardit.
Kiir, the new chairman of the SPLM/A, was himself sworn into office on 11 August, replacing John Garang who died in a helicopter crash on 30 July near the Uganda-Sudan border.
"The accession of two leaders who have a better - or at least less antagonistic - rapport with the SSDF [the government-aligned South Sudan Defense Force] leadership, presents an opportunity to solve the outstanding issue of how to deal with other armed groups in southern Sudan," David Mozersky of the International Crisis Group, told IRIN on Monday.
Like Garang, Kiir is a member of the Dinka community, south Sudan’s largest ethnic group. Machar’s participation is seen as a key factor in upcoming talks between the SPLM/A and Nuer militia that have been fighting on the government side.
"The fact that Riek came over from the government himself in 2002 will likely be a factor," Mozersky noted. "He is a Nuer, just like the leadership of the SSDF, and he has good personal relations with some of them.
"Many of the SSDF leaders served under Riek when they signed the Khartoum Peace Agreement [between the Sudanese government and various armed southern groups, represented by the SSDF] in 1997," Mozersky added.
Under the provisions of the Comprehensive Peace Agreement (CPA), signed between the Sudanese government and the SPLM/A on 9 January, all armed groups and militias in the south will either be integrated into the Sudanese Armed Forces or into the SPLM/A.
Disagreements among southern Sudanese leaders had, in the past, led to the creation of splinter groups, several of them supported by the Khartoum government. Observers feared that once Sudanese government troops withdrew from their southern positions, militias could try to fill the gap.
A previous round of talks in June and July between SSDF commanders Maj Gen Paulino Matip and Maj Gen Gordon Kong, and the SPLM/A - under Garang’s leadership - broke down amid recriminations.
"It is important for the implementation of the CPA that this issue is dealt with and the good relations between the leadership of the two movements is reason for optimism," Mozersky said.
On 13 August, the Sudanese news agency reported that Kiir and Matip had met in Khartoum to discuss the resumption of south-south talks, under the auspices of the former Kenyan President, Daniel Arap Moi.
Speaking to reporters after the meeting, Matip stressed the importance of uniting the southern ranks in order to tackle future challenges and invited Kiir to visit SSDF-controlled regions to help overcome misunderstandings.
Matip also called for the participation of the SSDF in southern security arrangements, the new southern government and the government of national unity.