Nikki Haley resigned Tuesday as the US ambassador to the United Nations, in the latest departure from President Donald Trump's national security team.
Meeting Haley in the Oval Office, Trump said that Haley had done a "fantastic job" and would leave at the end of the year.
"She told me probably six months ago," Trump told reporters, '"I want to take a little time off.'"
Haley stayed coy on her reasons for quitting, saying only that it was "important to understand when it's time to stand aside."
But she insisted that she was not planning to run for president herself against Trump in 2020, when she would likely be seen as a moderate Republican alternative.
The former governor of South Carolina, Haley took the job with little experience in foreign policy but quickly became the full-throated voice at the United Nations for the often unpopular agenda of Trump.
Haley has pressed for a hawkish line on Iran, justified US cuts to foreign assistance and earlier this year led the United States in bolting from the UN Human Rights Council, accusing it of bias against Washington and Israel.
At the recent UN General Assembly session, Haley took the highly unusual step for a senior diplomat of joining street protesters against Venezuelan President Nicolas Maduro, shouting from a megaphone that the leftist leader should leave office.
The daughter of Indian immigrants, Haley had long been seen as a rising star in the Republican Party which has eagerly sought female and ethnic minorities to broaden the appeal beyond its traditional white demographic.
Haley, whose given name was Nimrata Randhawa, was elected governor in 2010 of deeply Republican South Carolina and, during the 2016 campaign, had criticised Trump over his remarks on immigrants before their political reconciliation.
Haley's is the latest resignation in a turbulent White House, where Trump is already on his third national security adviser and second secretary of state before even the midterm elections.