SOUTH SUDAN (REUTERS) - The United States has lost trust in South Sudan's government for fueling the country's civil war and it must bring peace or risk losing support from Washington, US Ambassador to the United Nations Nikki Haley told the nation's President Salva Kiir.

Haley was the first senior member of President Donald Trump's administration to visit South Sudan, which spiraled into civil war in 2013, just two years after gaining independence from Sudan. She met one on one with Kiir for some 45 minutes.

"I let him know that the United States was at a crossroads and that every decision going forward was going to be based on his actions," Haley told reporters after the meeting in the capital Juba.

The United Nations has warned that the violence in South Sudan, which has forced some 4 million people to flee their homes, was providing "fertile ground" for a genocide. Kiir's government has denied UN allegations of ethnic cleansing.

Haley had to cut short a visit to a camp for South Sudanese displaced by the violence amid rowdy anti-Kiir protests.

"He understood that Americans were disappointed in his leadership in South Sudan, I made that very clear. And he understood that all the aid or help that he hopes will go forward is not a given," she said.

Haley did not elaborate on what further action Washington could take, but said that Kiir "got what I was trying to say."

On Monday she said Washington was considering how to pressure Kiir into peace, though noted that withdrawing aid may not work.

The Trump administration last month imposed sanctions on two senior South Sudanese officials and the former army chief.

"We have lost trust in the government and we now need to regain that trust and the only way to regain that trust is through the actions of taking care of all of the people," Haley told South Sudan's Eye Radio.