Donald Trump has accepted the resignation of UN Ambassador Nikki Haley, a White House official confirmed to CNN's Jim Acosta.
The resignation was first reported by Axios: “The timing of her departure is still unclear…Haley discussed her resignation with Trump last week when she visited him at the White House, these sources said. Her news shocked a number of senior foreign policy officials in the Trump administration.”
UPDATE: Trump made an announcement from the Oval Office after the news broke:

And then there's this: “United Nations Ambassador Nikki Haley's abrupt resignation on Tuesday came one day after an ethics watchdog group requested the State Department's inspector general investigate her acceptance of seven free flights aboard private jets from a trio of South Carolina businessmen. Haley, the former governor of South Carolina, listed the flights on her 2017 financial disclosure and asserted that each qualified for an exception based on her relationships with the businessmen.”