Nikki Haley announced she is resigning as the US ambassador to the UN, but denies she was preparing to run in the 2020 presidential election. Picture: Reuters/Jonathan Ernst

Washington - Nikki Haley announced on Tuesday she is resigning as the US ambassador to the United Nations, but the rising Republican star immediately denied she was preparing to challenge President Donald Trump in the 2020 presidential election.

Sitting beside Trump in the Oval Office, Haley said her 18-month stint at the United Nations had been "an honour of a lifetime" and said she would stay on until the end of the year.

A former governor of South Carolina who is the daughter of Indian immigrants, Haley is the highest profile woman in Trump's Cabinet and is often seen as a possible presidential candidate.

But she said in her resignation letter to Trump she would "surely not be a candidate for any office in 2020" and would support his re-election bid.

She told reporters in the Oval Office on Tuesday: "No, I am not running for 2020."

Haley said she did not have any future plans set and did not give a reason for quitting but Trump said she had told him some six months ago that she was thinking of leaving by the end the year "to take a little time off."

Haley's fast rise from a Southern state's legislature to the world stage suggests the 46-year-old may have greater political ambition.

Describing her UN tenure as "fantastic" and "incredible," Trump was effusive in his praise for Haley. He accepted her resignation and said he would name her successor within two or three weeks.

Haley has been the face of Trump's "America First" policy at the United Nations, steering the US withdrawal from several UN programmes and ardently defending his hard-line policies against Iran and North Korea over their nuclear programs.

But Haley has also sometimes distanced herself from Trump.

HIGH PRAISE FROM TRUMP

"She has done an incredible job. She is a fantastic person, very importantly, but she also is somebody that gets it," Trump said on Tuesday. "She's done a fantastic job and we've done a fantastic job together."

Haley's name has come up as possible Republican running mate in the last two presidential elections and she could also become a U.S. senator if fellow South Carolina Republican Lindsey Graham takes up a job in the Trump administration as if often speculated.

Trump suggested the two of them jointly announce Haley's departure in the Oval Office, an administration official said, noting that Haley was the only outgoing senior member of Trump’s administration to be feted in this way.

Returning the favour, Haley praised Trump and his family for their support.

She described Jared Kushner, Trump's son-in-law and aide, as "such a hidden genius that no-one understands" for his work on renegotiating the NAFTA trade deal and preparing a long-awaited Middle East peace plan that has not yet been released.

Haley was seen by her counterparts at the United Nations as a voice of clarity in a U.S. administration that often gave off mixed signals on foreign policy, diplomats say.

But she has been overshadowed in recent months by the appointments of Trump loyalist Mike Pompeo as secretary of state and hawk John Bolton as White House national security adviser.

Pompeo has led policy on talks with North Korea, while Bolton has taken the lead on trying to implement Trump's hard line against Iran.

Senator Robert Menendez, the top Democrat on the Senate Foreign Relations Committee, called Haley's resignation "yet another sign of the Trump Administration's chaotic foreign policy."

"I am deeply concerned about the leadership vacuum she leaves and the national security impact of her departure at this time of continued disarray," Menendez said in a statement.

Haley is the latest in a long line of high-profile departures from the administration, such as former secretary of state Rex Tillerson, who was fired in March, and Steve Bannon, Trump's chief strategist, who left in August 2017.

DOMESTIC CREDENTIALS

A convert to Christianity in her 20s after being raised a Sikh, Haley is popular with religious conservatives in the South.

Her status as a potential candidate for national high office was boosted in 2015 when she ordered the Confederate battle flag to be pulled down at South Carolina's state capitol grounds after a white supremacist gunman shot dead nine black worshipers at a church.

Haley has also applauded women who come forward to denounce sexual abuse or misconduct by men and said they should be heard, even if they are accusing Trump.

"Women who accuse anyone should be heard," Haley said last December after accusations against three members of Congress. "They should be heard, and they should be dealt with."

Referring to women whose accusations brought down powerful men, like movie producer Harvey Weinstein, Haley said: "I'm proud of their strength. I'm proud of their courage."

Haley has long taken a tougher public stance on Russia than Trump, who has sought better relations with President Vladimir Putin.

Haley raised eyebrows within the administration when she announced in April that Washington was going to impose sanctions on Moscow over its support of Syria's government. Trump then decided not to go ahead with the move.