United States Ambassador to the United Nations, Nikki Haley, has resigned, the latest departure from President Donald Trump’s national security circle, a senior White House official said Tuesday. 

The former South Carolina governor had served since January 2017. The reason for her departure was not immediately clear, and the official did not comment on what led to her resignation, according to CNBC.

Haley has represented the Trump administration at the UN amid several international crises, including efforts to dismantle North Korea’s nuclear program and ease conflict in Syria. 

She walked past reporters in the White House on Tuesday morning but did not comment. Trump then tweeted that he would have a “big announcement” with Haley in the Oval Office at 10:30 a.m. ET. 

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Haley is only the latest high profile Trump foreign policy official to leave the administration. Secretary of State Rex Tillerson and two national security advisors, Michael Flynn and H.R. McMaster, all served only brief stints. 

Haley, who has criticized the president on some foreign policy points, wrote a column in September denying that she penned an explosive New York Times op-ed. In the Times piece, an anonymous “senior administration official” said people working for Trump have worked to undermine his worst impulses on issues such as trade and Russia policy. 

Ethics groups had recently requested an investigation into Haley over whether she inappropriately accepted flights from South Carolina businessmen.