Sebastian Gorka, a White House national security aide whose hardline policy views attracted controversy, has become the latest official to leave Donald Trump's administration.
Mr Gorka said he had resigned from his position, but a White House official said he did not resign but "no longer works at the White House".
A former editor for the conservative Breitbart News website, Mr Gorka joined the administration as a counter-terrorism adviser. But he operated outside of the National Security Council and his exact responsibilities were not clearly outlined, though he was a prominent advocate for Mr Trump on television news.
Mr Gorka declined to discuss the reasons he left the White House, but pointed toward excerpts from his resignation letter posted by The Federalist website.
He wrote that "the individuals who most embodied and represented the policies that will 'Make America Great Again' have been internally countered, systematically removed, or undermined in recent months".
He criticised Mr Trump's recent speech on Afghanistan, writing, "The fact that those who drafted and approved the speech removed any mention of radical Islam or radical Islamic terrorism proves that a crucial element of your presidential campaign has been lost.
"The best and most effective way I can support you, Mr President, is from outside the People's House."
Mr Gorka is the latest in a string of White House exits. The most recent departure was chief strategist Steve Bannon, a key adviser in Mr Trump's general election campaign, but a contentious presence in a White House divided by warring staff loyalties.
He quickly returned to his post at Breitbart News, where he has said he intended to continue to advocate for his views.
Mr Bannon left the White House after the arrival of new chief of staff John Kelly, who had embarked on a personnel review of West Wing employees.
Mr Kelly has sought to impose discipline on a chaotic West Wing, limiting dissenting voices, restricting access to the president and "stacking the deck" on major decisions to guide Mr Trump toward an outcome.
Other recent exits include Mr Trump's first press secretary Sean Spicer, his first chief of staff Reince Priebus, and Anthony Scaramucci, who served for just 10 days as communications director.