Kurt Volker, the former U.S. special envoy to Ukraine who resigned his post last week after his name appeared in a whistleblower complaint about President Donald Trump's dealings with the Ukraine, will testify in the House's impeachment inquiry on Thursday, two committee aides told NBC News Tuesday.

Marie Yovanovitch, the former U.S. ambassador to Ukraine who was abruptly ousted from her post in May, will also sit for a joint deposition before the House Intel, Foreign Affairs and Oversight Committees on Oct. 11. Her deposition was originally scheduled for this Wednesday.

Volker resigned his post on Friday, after he was named in the whistleblower complaint and after Trump's personal lawyer, Rudy Giuliani, told reporters that Volker had encouraged him to speak to Ukrainian officials regarding former Vice President Joe Biden and his son Hunter Biden.

The whistleblower complaint said that the day after Trump spoke with Ukrainian President Volodymyr Zelenskiy and urged him to investigate Joe Biden, Volker traveled to the Ukrainian capital of Kyiv to meet Zelinskiy.

In a letter to House Democrats on Tuesday, Secretary of State Mike Pompeo pushed back against Democrats' request to interview five current and former State Department employees, including Yovanovitch and Volker.

"I am concerned with aspects of your request … that can be understood only as an attempt to intimidate, bully, and treat improperly the distinguished professionals of the Department of State," Pompeo wrote.

Democrats had also subpoenaed Pompeo for documents in an effort to find out more about the Trump administration's dealings with Ukraine and decision to withhold almost $400 million in aid to the country.

Later on Tuesday, the State Department’s Office of Inspector General scheduled a surprise briefing for Wednesday with staffers from a group of House and Senate committees on documents related to the State Department and Ukraine, multiple Congressional sources told NBC News.

Volker's resignation enables him to be much freer in what he can say before Congress, NBC News has reported.

Yovanovitch, who'd criticized Ukraine's top prosecutor for being too soft on corruption, was abruptly moved out of her post as Ukraine ambassador in May and recalled to Washington. The prosecutor she'd criticized, Yuriy Lutsenko, had reportedly told Giuliani that Biden and his son had engaged in wrongdoing while Biden was still vice president.

Lutsenko was later fired by Zelenskiy, a move Trump criticized in the July call.

"I heard the prosecutor was treated very badly and he was a very fair prosecutor," Trump said, according to a call summary.

Lutsenko told NBC News last week that he'd found no evidence the Bidens had done anything wrong.