The leader of South Sudan's rebels Riek Machar has cast doubt on the prospect of imminent face-to-face talks with President Salva Kiir despite pressure from the United States, a report said Sunday.

The independent Sudan Tribune website said it interviewed Machar in a rebel-held part of the country after a visit to the region by US Secretary of State John Kerry, who brandished the threat of sanctions if either Kiir or Machar fail to end the four-month-old civil war.

But the website quoted Machar as saying he was unhappy with Kerry's calls for direct talks between the two leaders and the formation of a transitional government in the country, which only won independence from Khartoum in 2011.

"I asked him (Kerry) what would be the purpose of transitional government? It would not be workable without a programme to implement before elections come," Machar was quoted as saying.

"We need to have a peace agreement first with a new constitution. Putting transitional government first is not realistic," he said, adding that face-to-face talks "may be counter-productive".

Kerry visited South Sudan on Friday and secured agreement from Kiir to meet with Machar in Addis Ababa in the coming days, with Ethiopian Prime Minister Hailemariam Desalegn acting as mediator.

Kerry also spoke with Machar, who "expressed openness to participating", according to State Department officials.

The Sudan Tribune, however, said Machar only signalled he was preparing to meet with the Ethiopian premier for "consultations".

The war in the world's youngest country has left thousands dead -- and possibly tens of thousands -- with at least 1.2 million people forced to flee their homes, many living in appalling conditions in overstretched UN bases and in fear of ethnic violence.

Aid agencies are also warning that South Sudan is on the brink of Africa's worst famine since the 1980s, while both Kerry and the UN's human rights chief have spoken out over their fears the country could slide towards a genocide.

The conflict erupted on December 15 with Kiir accusing Machar of attempting a coup. Machar then fled to the bush to launch a rebellion, insisting the president had attempted to carry out a bloody purge of his rivals.

President Barack Obama signed a decree last month authorising punitive sanctions, including the seizure of assets and visa bans, against anyone in South Sudan deemed to be threatening peace efforts.