UNITED NATIONS - The UN Security Council on Thursday welcomed the signing of a cease-fire agreement in South Sudan, calling it the first step toward comprehensive reconciliation and nation-building.

The council members called on all parties to immediately and fully implement the agreement as the first step in a longer process of ensuring durable peace and rule of law, national reconciliation, and the building of effective state institutions, the council said in a press statement.

Representatives of President Salva Kiir and former deputy president Riek Machar signed the agreement earlier Thursday on the cessation of hostilities, after three weeks of talks mediated by the Intergovernmental Authority on Development (IGAD) in the Ethiopian capital of Addis Ababa.

The ceasefire seeks to ease the political dispute in the world's youngest nation between Kiir and Machar, who was removed from office in July 2013 and later accused of attempting a coup.

Tensions escalated on December 15 into a full-scale conflict between forces loyal to the two men, making 494,000 people internally displaced and  83,900 others flocking into neighboring countries, over half of whom to Uganda, according to UN figures.

The 15-member council applauded the tireless efforts by IGAD to achieve the agreement and commended the African Union for its attempts to find a lasting solution to the ongoing crisis in South Sudan and its underlying causes, the statement said.

The council members voiced their unwavering support for the UN mission in South Sudan and strongly condemned the targeted and indiscriminate killings, widespread sexual and gender-based violence, recruitment and use of children, and arbitrary detention.

The council said that some of the conducts may amount to war crimes and crimes against humanity and that those responsible for human rights violations and abuses must be held accountable.