A former top aide to U.S. Secretary of State Mike Pompeo told impeachment investigators Wednesday that he quit last week in growing frustration over the politicization of the State Department, with the final straw President Donald Trump's ouster of the well-regarded American ambassador to Ukraine, Marie Yovanovitch.

In hours of congressional testimony, Michael McKinley decried the agency's unwillingness to protect career diplomats like Yovanovitch from political pressure.

McKinley's statements, recounted by people familiar with his closed-door testimony before the House of Representatives Intelligence Committee, are the latest in a string of unflattering accounts about the behind-the-scenes operations of the country's foreign policy and national security agencies.

McKinley has served as the U.S. ambassador in four countries, and he had other global postings before returning to Washington as an aide to Pompeo.

His testimony, along with that of others, has helped buttress the account of an unnamed whistleblower in the U.S. intelligence community about a late July phone call between Trump and Ukraine President Volodymyr Zelenskiy. In the call, Trump asked the Kyiv leader for "a favor" - that Ukraine investigate one of Trump's top Democratic challengers, former Vice President Joe Biden, and the business dealings of his son, Hunter Biden.

Both Bidens have denied wrongdoing, but Hunter Biden, 49, said this week he used "poor judgment" in agreeing to serve on the board of a Ukrainian energy company because it had become a political liability for his father.

Trump has described the call with Zelenskiy as "perfect," and denied any wrongdoing.

Yovanovitch testified last week that Trump dismissed her based on "unfounded and false claims" after Trump's personal attorney, former New York Mayor Rudy Giuliani, assailed her performance in Kyiv.

According to a rough recounting of the July conversation supplied by the White House, Trump told Zelenskiy, "The former ambassador from the United States, the woman, was bad news and the people she was dealing with in the Ukraine were bad news, so I just wanted to let you know that. The other thing, there's a lot of talk about Biden's son, and that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the attorney general would be great. Biden went around bragging that he stopped the prosecution so if you can look in to it ... it sounds horrible to me."

Trump continued Wednesday to attack the impeachment hearings against him.

On Twitter, he contended, "Republicans are totally deprived of their rights in this Impeachment Witch Hunt."