Nikki Haley said her country was giving up on President Kiir for failing to end the violence.
South Sudan has protested against a top United States diplomat's remark questioning President Salva Kiir's suitability to rule the young the nation.
The president's spokesperson Ateny Wek Ateny said the comment by Ms Nikki Haley to the United Nations was demeaning, outrageous and an insult to the South Sudanese sovereignty.
Ms Haley on Wednesday told the UN Security Council that her country was giving up on President Kiir for failing to end the violence in the war-torn nation.
She also said President Kiir had become an unfit partner to the US for prolonging the war and the suffering of the people of South Sudan.
But Mr Ateny said it was not the responsibility of the US envoy to talk about the fitness of the South Sudan leader, as that was the preserve of the citizens.
“That is outrageous for a foreign diplomat to talk about the fitness of the President of another country.
“It is not the business of Nikki Haley to say President Kiir is unfit or fit to rule South Sudan. This is the business of South Sudanese people and they are the ones to decide, but not Haley,” Mr Ateny said.
“President Kiir is fit to govern South Sudan and the people of South Sudan know that,” Mr Ateny told Africa Review on phone.
Ms Haley, the US media reported, slammed President Kiir for failures to respect last December ceasefire deal as well as his promotion of three army generals who were sanctioned by the UN for leading the slaughter of civilians in the four-year civil war.
The three contentious generals are Santino Deng Wol, head of ground forces, Marial Chanuong, head of army operations, training and intelligence and Jok Riak, deputy chief of defence.
“Attempts to ease the suffering of South Sudan’s people are failing, not just despite the leadership of South Sudan, but because of it,” Ms Haley was quoted saying.