The United States and Britain pushed for UN sanctions against South Sudan's government Tuesday, over its failure to sign a peace deal to end a brutal two-year civil war.

South Sudan rebel chief Riek Machar met a Monday deadline to sign the power-sharing agreement, but President Salva Kiir only initialled part of it and said he would return to the table in early September to finalise the accord.

US National Security Advisor Susan Rice accused Kiir's government of a "failure of leadership" and said it had "squandered" another opportunity to end a conflict that has killed tens of thousands and which has plunged the world's youngest nation into chaos.

"There must be consequences for those who continue to stand in the way of peace," she said, calling for UN Security Council sanctions if the government does not sign the deal within 15 days.

READ: Groups differ on South Sudan sanctions

On a recent visit to East Africa, US President Barack Obama threw his personal weight behind efforts to foster peace in a country midwifed into existence by Washington, but then, critics say, abandoned at birth.

The latest peace accord was brokered by the eight-nation East African IGAD bloc, bolstered by the UN, the European Union, the African Union, China and other players — including Britain and the United States.

At the UN headquarters in New York Tuesday countries weighed their next move.

"If the government will not sign up to the IGAD-plus deal, then we must all be firm on our next steps," British Deputy Ambassador Peter Wilson told the 15-member Security Council.

"We cannot sit by while leaders fight and their people's suffering grows."

More time for Kiir

The council last month imposed sanctions on six commanders — three from the government forces and three from the rebels — the first to be blacklisted by the United Nations over the conflict.

A travel ban and an assets freeze were slapped on the six men and the council is considering adding new names to the sanctions list, as well as an arms embargo.

But China, which has oil interests in South Sudan, said the government should be allowed more time to come on-board.

"The best solution would be to reach an agreement," said Chinese Ambassador Liu Jieyi.

UN Secretary-General Ban Ki-moon welcomed the signing of the deal by the rebel leader and expressed "his strong hope that President Kiir will sign the agreement by the end of the 15-day deadline."

The Security Council is due to discuss sanctions on South Sudan at a meeting next Tuesday.

South Sudan has been torn by fighting between forces loyal to Kiir and rebels allied with Machar, his former deputy, since December 2013 and the violence has imploded along ethnic lines.

Nearly 70 per cent of the country's population is facing food shortages while nearly 200,000 terrified civilians are sheltering in UN bases.