IGAD has been striving to broker the two warring parties in an effort to avert a civil war as the conflict spreads to other regions of the country. On Monday, Ugandan President Yoweri Museveni warned rebel leaders to agree to a ceasefire or face defeat.
"We gave him [Machar] four days and [agreed that] if he doesn't [comply with the agreement], then we shall have to go for him. That is what we agreed on", Museveni told reporters in Juba.
Museveni, a close ally of Kiir, also admitted that he sent Ugandan troops to "help restore hope" in South Sudan, confirming his military's involvement in the conflict.
He did not, however, elaborate on how the region intends to defeat a rebellion, which has quickly spread to the country's key towns.
But Machar's group, in a statement, warned that any aggression from the Ugandan army could compromise IGAD's attempt to broker a peace deal between the conflicting parties.
East African leaders say the conflict in South Sudan is a threat to the stability of the region and the current political crises needs an urgent solution.