Mr Trump has also pardoned controversial former sheriff Joe Arpaio
President Donald Trump's deputy assistant, Sebastian Gorka, has left the White House.
The national security and counter-terrorism expert told the Associated Press he had resigned from his position.
But a White House official, speaking on condition of anonymity, said Mr Gorka did not quit but "no longer works at the White House", suggesting he may have been fired.
Mr Gorka declined to discuss the reasons he left, but in his letter of resignation he expressed dissatisfaction with the Trump administration.
Posted by The Federalist website, Mr Gorka 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".
Mr Gorka was a national security adviser to Breitbart News under Steve Bannon, who also left the Trump administration a week ago.
The 46-year-old had been accused of ties to far-right groups and his claimed counter-terrorism knowledge was repeatedly questioned by peers.
Meanwhile, Mr Trump has granted his first presidential pardon to former Arizona sheriff Joe Arpaio.
The former lawmaker was convicted of criminal contempt in a case involving his department's racial profiling policy.
President Trump has frequently praised Mr Arpaio for his hardline stance on immigration.
In another development, President Trump has issued a directive ordering the US military to reject openly transgender people as new recruits from March next year.
The president has ordered that a criteria be set on whether transgender individuals, currently serving, should be allowed to continue in their roles.
The Obama administration in June 2016 had changed longstanding policy, declaring that troops could serve openly as transgender individuals, and set a July 2017 deadline for determining whether transgender people could be allowed to enter the military.
US Defence Secretary Jim Mattis delayed that to January 2018, but Mr Trump has now instructed him to extend it indefinitely.