US President Donald Trump has accepted the resignation of his UN ambassador Nikki Haley.
Ms Haley, who has held the post since November 2016, will be leaving the role at the end of the year.
The former South Carolina governor, who is believed to hold aspirations for higher office, ruled out running for president in 2020 at the White House on Tuesday.
She added that she will be supporting Mr Trump.
Ms Haley has not given a reason for her resignation.
The president called her a "very special person" and added that she told him six months ago that she wanted to take some time off.
Big announcement with my friend Ambassador Nikki Haley in the Oval Office at 10:30am.
— Donald J. Trump (@realDonaldTrump) October 9, 2018
Mr Trump said that together they had "solved a lot of problems".
The president added that he hoped she could come back to his administration in another capacity.
He said her successor would be named in the next two or three weeks.
Ms Haley wrote an article in the Washington Post in September in which she discussed her policy disagreements but also her pride in working for Mr Trump.
She wrote: "I proudly serve in this administration, and I enthusiastically support most of its decisions and the direction it is taking the country.
"But I don't agree with the president on everything."
The president had earlier tweeted on Tuesday that a "big announcement" was due with his "friend" at 10.30am local time (3.30pm UK time).
Ms Haley had served as the first female governor of South Carolina before she was appointed to the UN ambassador role.
She was an outspoken critic of Mr Trump during his 2016 presidential campaign – a stance that effectively disqualified other candidates for top administration positions.
The daughter of Indian immigrants, she alluded to Mr Trump in denouncing "the siren call of the angriest voices" who disrespected America's immigrants.
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Mr Trump tweeted that "the people of South Carolina are embarrassed by Nikki Haley".
Ms Haley is the latest in a long list of senior staff members to leave the Trump administration, including former secretary of state Rex Tillerson, who was fired in March.