US Border Patrol Chief Mark Morgan, a former longtime FBI agent, has left the agency, sources familiar with his departure told Reuters on Thursday.

Morgan told employees on a phone call Thursday morning that he was not resigning but had been asked to leave by the Trump administration, said a Department of Homeland Security official who spoke on the condition of anonymity.

A statement provided by acting commissioner Kevin K. McAleenan didn't provide any details of the departure.

"I want to thank Mark Morgan for his unwavering dedication to our border security mission, and recognize his life-long career in service to the nation," McAleenan said in the statement.

Morgan's departure comes a day after President Donald Trump signed executive orders directing the construction of a wall along the Mexican border and the hiring of 5,000 additional border patrol agents.

The border patrol union, which endorsed Trump's presidential campaign and applauded his executive order, had been critical of Morgan. The union criticized Morgan for supporting former President Barack Obama's plans to safeguard certain undocumented immigrants from deportation.

The executive board of the National Border Patrol Council called Morgan arrogant and a "disgrace to the Border Patrol" in an opinion piece published by conservative website Breitbart News on Nov. 30.

Unlike many border patrol officers, Morgan did not climb the chain of command within the agency but was appointed from the FBI.