UPDATE 2-U.S. Ambassador to United Nations Nikki Haley quits - source

U.N. Ambassador Nikki Haley has resigned, a person familiar with the situation said on Tuesday, and the White House said she would appear with President Donald Trump at an event later in the morning.

Trump accepted the resignation, Axios news site reported, citing two sources familiar with the matter. Haley discussed her resignation with Trump last week when she visited him at the White House, these sources said.

It was not immediately clear why she quit.

Trump wrote on Twitter that there would be a "Big announcement with my friend Ambassador Nikki Haley in the Oval Office" on Tuesday morning.

White House spokeswoman Sarah Sanders said the two would meet in the Oval Office with news media present.

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.

Haley, who has been one of Trump's most trusted advisers, would not confirm the report to Reuters when asked about it during a visit to the White House on Tuesday.

Earlier this year, Haley, 46, told Reuters that, "Every day I feel like I put body armour on," to protect U.S. interests at the United Nations.

Haley was governor of South Carolina when she was tapped by Trump to be the U.S. envoy to the United Nations. Elected as the state's first female governor in 2010, she was in her second four-year term when Trump picked her.  

(With inputs from agencies.)