The United States, a key supporter of South Sudan, said it was calling back its ambassador for consultations as it shows frustration with duelling leaders' failure to form a government.

Secretary of State Mike Pompeo wrote on Twitter that he called back the ambassador "as we re-evaluate our relationship with the government of South Sudan."

"We will work with the region to support efforts to achieve peace and a successful political transition in South Sudan," he said.

The United States, which contributes about USD 1 billion a year in mostly humanitarian aid for the young country, has been especially vocal in its exasperation over the lack of progress in South Sudan.

South Sudan President Salva Kiir and rebel chief Riek Machar fell out in 2013 -- two years after the largely Christian nation won independence from Sudan with strong US support -- sparking a conflict that has left hundreds of thousands dead.

The two leaders missed a November 12 deadline to form a unity government. African mediators gave them another 100 days, the second extension.

The US ambassador in Juba, Thomas Hushek, will hold talks in Washington on the re-evaluation of the relationship, State Department spokeswoman Morgan Ortagus said.

Tibor Nagy, the assistant secretary of state for African affairs, said earlier this month that he believed Kiir and Machar had grown content with the status quo.

"The international community is providing the food, the medicines, basically all of the human needs that are the responsibilities of governments to do. They're basically sitting back," he said.