Sudan has urged the warring parties in neighbouring South Sudan to iron out all outstanding disputes ahead of the set date for the formation of a transitional unity government.

Mohamed Hamdan Daqlu, deputy head of the Sudan Sovereign Council, said on Wednesday that the 100 day-extension of the pre-transitional period agreed upon by President Salva Kiir and Riek Machar in November was fast running out amid a deadlock.

Daqlu's remarks came following a meeting with the two South Sudan leaders in Juba.

"This meeting is part of the previous meetings," Daqlu told journalists in Juba after meeting Kiir together with Machar. "We need to focus on the implementation of the South Sudanese peace."

Daqlu is the latest leader to make the peace call to South Sudan's factions after South African Deputy President David Mabuza earlier this month.

South Sudan plunged into chaos in December 2013 sparked by a feud between President Kiir and Machar.

President Kiir accused Machar, his then deputy, of plotting a coup against his rule. Machar denied the allegations but then went on to mobilize a rebel force to fight the government.

The war has killed hundreds of thousands and displaced millions, making South Sudan Africa's biggest refugee crisis.

Under the auspices of regional bloc IGAD, Sudan, Ethiopia and Kenya played key roles in mediation of the revitalized peace agreement.