The latest round of peace talks between South Sudan's two rival leaders has failed to strike a deal on the vital issue of internal boundaries, mediators said on Friday.

South Sudan President Salva Kiir and rebel leader Riek Machar held fresh talks Wednesday in a bid to break a deadlock over states and internal borders in the troubled country, in order to form a unity government next month.

South Sudan President Salva Kiir and opposition leader Riek Machar agreed Tuesday to form a unity government by a February deadline, as they face international censure over faltering peace talks.

South Sudan President Salva Kiir and rebel leader Riek Machar will hold rare face-to-face talks Thursday in Uganda, their representatives said, as time runs out for the rivals to form a power-sharing government.

South Sudan’s main opposition accused the government on Wednesday of failing to push through a peace deal and called for a six-month delay in the formation of a unity administration.

South Sudan's exiled rebel leader Riek Machar arrived in Juba on Saturday in a bid to salvage a peace deal with less than a month to go until he is to form a government with President Salva Kiir.

South Sudan's exiled rebel leader Riek Machar arrived in Juba on Monday for the first time in a year to meet his rival, President Salva Kiir, and try salvage a stalled peace agreement.

South Sudan's exiled opposition leader Riek Machar says he is ready to meet his rival President Salva Kiir to talk peace if certain conditions on his freedom were met.

The UN said Wednesday that conflict had intensified in a region of South Sudan since a peace deal was signed, with hundreds of civilians raped or murdered by warring factions.

South Sudan's rival parties began two days of talks in Addis Ababa on Thursday in a bid to salvage a peace deal, with just days left before a unity government is meant to be formed.

South Sudan opposition leader Riek Machar is not ready to return to Juba and wants to postpone the formation of a unity government until security issues are resolved, an official from his party said Wednesday.

South Sudan's warring rivals President Salva Kiir and rebel leader and former vice president Riek Machar will attend a retreat at the Vatican this week "to work for a future of peace", the Holy See said.