The signing is scheduled to take place in Juba tomorrow with Kiir to be joined by the leaders of Kenya, Uganda, Sudan and Ethiopia for a mini-summit.
South Sudan's rebel leader Riek Machar, a former vice president, met a deadline to sign the agreement on August 17.
UN envoy and mission chief Ellen Margrethe Loej told the Security Council that tomorrow's signing would be a "hopefully positive development" but that many hurdles lie ahead to implement the agreement.
"Peace, stability and prosperity will not come to South Sudan overnight," she warned.
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The world's youngest nation, South Sudan has been torn by fighting between forces loyal to Kiir and rebels allied with Machar since December 2013 and the violence has imploded along ethnic lines.
Loej, who heads the UN peacekeeping mission in South Sudan, said attention must turn to the inter-ethnic fighting which in some states is just as violent as the struggle opposing the two camps.
Nearly 70 per cent of the country's population is facing food shortages and some 200,000 terrified civilians are sheltering in UN bases.
The UN envoy spoke to the 15-member council amid negotiations on a US draft resolution imposing an arms embargo and targeted sanctions on South Sudan over the failure to sign the peace deal.
Russia, a veto-wielding member of the council, said there would be no need to adopt the resolution if Kiir signs the deal.
The United Nations released a report by a panel of experts detailing arms supplies from China to government forces along with shipments of Russian-made and Israeli-made weapons, possibly supplied through regional countries.
South Sudan has over the past year signed security agreements with Egypt and Uganda, the report noted.
Kiir's government had budgeted USD 850 million for arms contracts from January to July 2014, the report said, although the experts cautioned that the weaponry listed for purchase was not delivered.
The experts called for a full arms embargo, stating that the "continuing resupply of arms and ammunition on both sides has been instrumental in the continuation and escalation of the war to its current scale.