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 on Friday said a new delay to set up a power-sharing government was "endurable" as he vowed to honour a peace deal funding pledge for a unified army.

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 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.

Sudanese Prime Minister Abdallah Hamdok will attend peace talks in the South Sudan capital Monday with rebel leaders from several Sudanese states, said official sources in Juba. "Tomorrow's meeting is to mark the launching of Sudan's peace talks," Ateny Wek Ateny, spokesman for South Sudan's President Salva Kiir, told AFP Sunday.

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.

The UN Security Council's decision to renew an arms embargo on South Sudan was "anti-peace" and will weaken the government's fight against rebels, a senior minister told AFP on Friday.

South Sudan's President Salva Kiir said Wednesday that a six-month delay agreed with his rivals for the formation of a unity government was not enough time to resolve outstanding issues.