U.S. Ambassador to Brazil Michael McKinley speaks during the first edition of the International Brazil Air Show at Tom Jobim international airport, known as Riogaleao, in Rio de Janeiro, Brazil, on March 29, 2017. / AFP PHOTO / YASUYOSHI CHIBA        (Photo credit should read YASUYOSHI CHIBA/AFP/Getty Images)
PHOTO: YASUYOSHI CHIBA/AFP/AFP/Getty Images
U.S. Ambassador to Brazil Michael McKinley speaks during the first edition of the International Brazil Air Show at Tom Jobim international airport, known as Riogaleao, in Rio de Janeiro, Brazil, on March 29, 2017. / AFP PHOTO / YASUYOSHI CHIBA (Photo credit should read YASUYOSHI CHIBA/AFP/Getty Images)
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(CNN) —  

A senior adviser to Secretary of State Mike Pompeo has resigned, two sources familiar with the matter told CNN on Thursday.

The adviser, Michael McKinley, had been a diplomat for more than three decades and served as ambassador to Peru, Colombia, Afghanistan and, most recently, Brazil until he was appointed to his advising role in May 2018, according to his department biography. McKinley was a trusted adviser of the top US diplomat, who recruited him as an adviser and liaison to the career service and traveled with him on official trips frequently.

In a farewell email to his State Department colleagues obtained by CNN on Friday, McKinley wrote that he was “leaving the department to pursue other opportunities, wherever they might lead.

“The decision is personal, its time after 37 years with the Department.”

Asked about McKinley’s departure in an interview in Nashville on Friday, Pompeo said his adviser “wanted to go on and begin the next phase of his life.”

“You know I don’t talk about personnel matters, but I’ve known Mike McKinley for quite some time, and he’s been with the State Department for 37 years. He told me for lots of good and sufficient reasons for him and his family he wanted to go on and begin the next phase of his life,” he told WKRN-TV, a CNN affiliate.

The departure comes as Pompeo faces increasing pressure from House investigators over his recently revealed presence on a phone call in which President Donald Trump pressured Ukranian President Volodymyr Zelensky to investigate former Vice President Joe Biden – Trump’s potential Democratic rival – and Biden’s son Hunter. There is no evidence of wrongdoing in Ukraine by Joe or Hunter Biden.

The Washington Post first reported McKinley’s resignation, adding that it came in the midst of growing discontent at the State Department in response to Pompeo’s perceived lack of support for department officials caught up in investigations linked to Ukraine.

A person familiar with the situation told the Post that McKinley saw Pompeo as benefiting the department – compared to previous Secretary of State Rex Tillerson – for fostering differing viewpoints and halting Tillerson’s freeze on promotions and limits on department employees’ spouses working abroad.

The Post also reported that while McKinley was not working directly on Ukraine policy, he disapproved of what he saw as Pompeo’s insufficient public support for department officials implicated in Trump’s dealings with Ukraine and subsequently called to testify in House Democrats’ investigation.

McKinley was involved in several policy spheres, holding key roles in foreign policy efforts in Venezuela, Mexico and Afghanistan, the Post reported – the last a key international focus after Trump revealed last month that Taliban leaders had been slated to travel to the US for secret peace talks but that the meeting was canceled as he called off peace talks with the militant group entirely.

CNN’s Jennifer Hansler contributed to this report.