File photo shows South Sudanese President Salva Kiir (R) and former South Sudanese first Vice-President Riek Machar (L) in Juba, capital of South Sudan, on April 26, 2016. Riek Machar was then sworn in as the country's first vice president. (Xinhua/Denis Elamu)

JUBA, Dec. 25 (Xinhua) -- A cease-fire between government and several rebel groups in war-torn South Sudan went into effect on Sunday, aiming to revive a stalled 2015 peace deal and end four years of civil war.

The South Sudanese government and numerous opposition factions on Thursday signed a cessation of hostilities agreement in Addis Ababa, Ethiopia and it began on Dec. 24.

The cease-fire agreement brokered by the Intergovernmental Authority on Development (IGAD), an east Africa regional bloc, asked the warring parties to stop military operations, demands that forces remain in their bases and further called for release of political detainees.

South Sudanese President Salva Kiir on Saturday directed the leadership of the army to comply with the agreement.

Riek Machar, leader of the main rebel group, Sudan People's Liberation Army-in Opposition (SPLA-IO) on Friday ordered his forces to remain in their bases and cease confrontations.

It remains unclear whether the rival factions would this time respect the latest cessation of hostilities agreement as they previously violated several others.

The United Nations Mission in South Sudan (UNMISS) on Friday called on all parties to respect the pact and work collectively to ensure durable peace in the east African nation.

"The Agreement on Cessation of Hostilities, Protection of Civilians and Humanitarian Access is an important first step in the revitalization of the peace process," UNMISS said in a statement.

"UNMISS is committed to supporting the peace process in line with its mandate through advancing reconciliation efforts, strengthening conflict resolution mechanisms, building national cohesion and engaging in regional and international peace initiatives," it added.

IGAD and the international community are betting on the High-Level Revitalization Forum convened in Addis Ababa as the last push to end the devastating conflict in the world's youngest nation.

South Sudan has been embroiled in four years of conflict that has taken a devastating toll on the people, and creating one of the fastest growing refugee crisis in the world.

A peace deal signed in August 2015 between the rival leaders under UN pressure led to the establishment of a transitional unity government in April, but was shattered by renewed fighting in July 2016.

The UN estimates that about 4 million people have been displaced internally and externally.

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