U.N. Secretary General Ban Ki-moon spoke with South Sudan's president and the top rebel leader Monday, urging them to quickly establish their transitional government, seen as a vital step to ending the country's civil war.
The telephone calls came as rebel leader Riek Machar prepared to return to the capital, Juba, to be sworn in as first vice president and set up the government with President Salva Kiir.
The United Nations says Ban commended Kiir for his decision to welcome Machar to Juba and called on him to implement security arrangements outlined in the peace deal that Kiir and Machar signed last August.
Ban welcomed Machar's decision to return to Juba and urged him to work with the president to prevent further violence.
Machar had been expected to arrive Monday in Juba, but a spokesman for the rebel advance team said a lack of flight clearance will delay Machar until Tuesday.
South Sudan erupted into civil war in December 2013, six months after Kiir fired Machar as vice president. Two and a half years of fighting has killed tens of thousands and displaced more than two million South Sudanese from their homes.
Aid agencies warn the country is at the edge of a humanitarian catastrophe, caused by poor harvests and food shortages since the start of the war.