President Salva Kiir and the SPLM/A-IO leader Dr Riek Machar on Thursday in Kampala, Uganda agreed to form the transitional government on February 22, 2020.
The extension of the pre-transitional period for three more months is to implement security arrangements and the issue of the number of states and their boundaries.
The regional groups will work with South Sudan and Intergovernmental Authority on Development or IGAD during the 100-day extension.
The expansion came during Kampala Summit, headed by Ugandan President Yoweri Kaguta Museveni, Kenyan Special Envoy to South Sudan and the head of Sudan Sovereign Council.
Earlier President Kiir assured the UN Security Council and the public that the government will be formed on November 12th, but Dr Machar demanded for three months to resolve outstanding issues to avoid the repeat of July 2016.
The signatories to the agreement were expected to proceed to Addis Ababa, Ethiopia for the IGAD meeting on the outstanding issues.
President Kiir is expected to arrive in Juba on Friday to address the public.