WASHINGTON (Reuters) - A senior adviser to U.S. Secretary of State Mike Pompeo has resigned, with the State Department mired in a controversy over the Trump administration’s dealings with Ukraine that has prompted an impeachment inquiry targeting the president.

FILE PHOTO: Secretary of State Mike Pompeo, left, sits down for a meeting with Japanese Foreign Minister Taro Kono, in Osaka, Japan, Friday, June 28, 2019, during the G-20 summit. At right is the secretary's senior adviser Michael McKinley. Jacquelyn Martin/Pool via REUTERS

Michael McKinley, a career diplomat who has served as Pompeo’s policy adviser since May 2018 following previous posts as ambassador to Brazil, Afghanistan, Colombia and Peru, said in a note to staff made available on Friday that his departure after 37 years with the department was “personal.”

He said he was “leaving the Department to pursue other opportunities, wherever they may lead.”

McKinley departure comes as the congressional impeachment inquiry heated up.

While McKinley has not been directly involved with Ukraine, he has served as a conduit between Pompeo’s office and career staff, according to the Washington Post, which first reported his resignation on Thursday night.

House Democrats have issued a subpoena to Pompeo for documents as part of their probe, which centers on a July 25 call Trump made to Ukrainian President Volodymyr Zelenskiy in which he pressed for an investigation into former U.S. Vice President Joe Biden, a leading Democrat seeking the party’s nomination in the 2020 presidential election.

Lawmakers have said Pompeo was not cooperating with the subpoena.

The State Department earlier this week blocked testimony by the U.S. ambassador to the European Union, Gordon Sondland, who was then subpoenaed to appear on Oct. 16.

Former U.S. ambassador to Ukraine, Marie Yovanovitch, told the impeachment inquiry that Trump ousted her based on “unfounded and false claims” after she had come under attack by Trump personal lawyer Rudy Giuliani.

Pompeo was asked about McKinley’s resignation in an interview with Joel Ebert of The Tennessean on Friday and brushed off reports that the adviser was leaving because his boss had failed to protect employees.

“I protect every single State Department employee,” Pompeo said. “When the State Department employees are doing things right, when they’re behaving in ways that are consistent with the rule of law and working on President Trump’s and America’s mission, I’ll always stand with them; I’ll always have their back.”

In another interview, with WKRN-TV in Nashville, Pompeo said McKinley “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.”

Pompeo declined to elaborate.