The spokesman for US President Donald Trump's National Security Council plans to leave the administration, the White House has confirmed.
Michael Anton, who also worked for former President George W Bush's National Security Council and is a former BlackRock managing director, is leaving after serving under Trump for more than a year, White House press secretary Sarah Sanders said in a statement.
Trump telephoned Mr Anton on Sunday to thank him for his service, a White House official said.
The White House did not offer a reason for Anton's departure or say exactly when he would leave.
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The announcement comes a day before Trump's third national security adviser formally takes up his post, with the departure the latest in a string of staff changes in the administration.
Mr Anton told Politico he planned to join Hillsdale College's Kirby Center in Washington as a writer and lecturer.
He published a widely read essay in conservative circles before the 2016 election under a pseudonym that compared the presidential race to the doomed September 11, 2001, flight that crashed in Pennsylvania after passengers charged the cockpit.
"2016 is the Flight 93 election: charge the cockpit or you die," he wrote.
"To compound the metaphor: a Hillary Clinton presidency is Russian Roulette with a semi-auto. With Trump, at least you can spin the cylinder and take your chances."
Last month, Trump said he was replacing HR McMaster as national security adviser with John Bolton, a hawk who has advocated using military force against North Korea and Iran.
A week earlier, Trump fired Rex Tillerson as secretary of state and nominated Central Intelligence Agency Director Mike Pompeo to replace him.
