"Neither the government nor the opposition has been consulted about reports presented by IGAD to the African Union Peace and Security Council," Machar told The Anadolu Agency on Friday.
"We cannot reach a solution to the [South Sudan] crisis away from the government and the opposition," he said.
"We reject any custodianship of South Sudan," Machar added.
The IGAD regional grouping, which is based in Djibouti, has postponed a planned summit to discuss South Sudan's year-long conflict until Saturday.
South Sudan, which became independent in 2011, has been shaken by violence since late 2013, when President Salva Kiir accused Machar, his sacked vice-president, 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 people have been uprooted from their homes, and hundreds of thousands now seek shelter in refugee camps across the country.
In recent months, the warring camps have held on-again, off-again IGAD-backed peace talks in Addis Ababa.
The talks, however, have largely failed to produce any breakthroughs.