South Sudan's government said on Thursday that a threat of US sanctions if the country fails to deliver on a peace deal should be directed at rebels in the country rather than the presidency.

South Sudan descended into civil war in December 2013 following a row between President Salva Kiir and his sacked deputy Riek Machar.

Both sides, under pressure from Washington, the United Nations and African Union, signed an initial peace deal in August and agreed to share out ministerial positions in January. But that deal has broken down repeatedly.

On Wednesday, US Secretary of State John Kerry said Kiir and Machar - who was re-appointed to his old post this month - would face individual sanctions if they did not deliver on the deal, warning of a "critical moment for South Sudan's survival".

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A spokesman for Kiir, Ateny Wek Ateny, said the threat "must be aimed at those who are putting impediments into the implementation process ... The problem is not with Salva Kiir".

He said Kiir was ready to form a transitional government "tonight" if Machar's side, the SPLM/A-IO, submitted the names.

James Gatdet Dak, spokesman for the SPLM/A-IO, said Machar's side was "ready to deliver" and Kerry's comments were helpful.

"This is a good statement because it can put pressure on the two parties. But on our side we are ready to deliver, it is the government that has been violating the peace agreement," he told Reuters.

Machar troops in Juba

This week, both sides agreed to let troops allied with Machar to return to the capital Juba for the first time since the conflict broke out, and a spokesman for Machar said he planned to return to Juba next week.

UN Secretary General Ban Ki-moon is due to visit South Sudan on Thursday and meet Kiir, who has led the country since independence from Sudan in 2011.

In a sign of lingering violence, aid group Medicins Sans Frontieres said on Wednesday its camp in Pibor had been attacked and looted, wounding 35 people and forcing more than 1,000 others to seek shelter at a base of the UN peacekeeping mission.