Nikki Haley has resigned as the U.S. ambassador to the United Nations. It is not immediately clear what has prompted the move. Par Laurel Wamsley

Haley informed her staff Tuesday of the news, NPR's Michele Kelemen reports. Haley is scheduled to appear with President Trump at the Oval Office this morning.

Haley, the former governor of South Carolina, has been a fierce advocate for Trump's policies at the U.N.

« I proudly serve in this administration, and I enthusiastically support most of its decisions and the direction it is taking the country, » Haley wrote in The Washington Post last month, after The New York Times published an anonymous op-ed critical of the Trump administration. « But I don't agree with the president on everything. »