Philip Reeker, the acting assistant secretary of European and Eurasian affairs, testified Saturday before the House committees conducting the impeachment inquiry in a closed deposition. Reeker is the latest in a line of current and former administration officials testifying about their knowledge of a July 25 call where President Trump asked the Ukrainian president to investigate former Vice President Joe Biden and his son, Hunter Biden.

An official working on the impeachment inquiry told CBS News that Reeker received a subpoena to appear.

"In light of an attempt by the State Department to direct Ambassador Philip Reeker not to appear for his scheduled deposition, and efforts by the State Department to also limit any testimony that does occur, the House Intelligence Committee issued a subpoena to compel his testimony. As is required of him, Ambassador Reeker is now complying with the subpoena and answering questions," the official said.

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CBS News' Margaret Brennan has reported that Reeker had raised concerns about a smear campaign against former U.S. Ambassador to Ukraine Marie Yovanovitch spearheaded by the president's personal attorney, Rudy Giuliani, alleging that Yovanovitch was disloyal to Mr. Trump. Yovanovitch was removed from her position in May.

The Wall Street Journal reported Saturday that Reeker was set to tell congressional investigators that Secretary of State Mike Pompeo and other top officials had stymied Reeker's efforts to show support for Yovanovitch.

The impeachment inquiry intensified over the past week, as Bill Taylor — the top U.S. diplomat in Ukraine — testified on Tuesday that U.S. aid to Ukraine was explicitly tied to the country's willingness to investigate Mr. Trump's political rivals.

In his 15-page opening statement, Taylor described a concerted effort to use U.S. leverage to get Ukrainian President Volodymyr Zelensky to commit to opening investigations into debunked allegations of Ukrainian interference in the 2016 U.S. election, as well as the gas company Burisma, which had hired former Biden's son in 2014. Taylor said these efforts came via an "irregular, informal channel of U.S. policy-making" consisting of Rudy Giuliani, then-special envoy Kurt Volker, Energy Secretary Rick Perry and U.S. Ambassador to the E.U. Gordon Sondland.

House Republicans also ramped up their criticism of the impeachment inquiry, arguing that depositions should be public. On Wednesday, more than 20 Republican lawmakers refused to leave a secure hearing room to protest closed-door impeachment proceedings, delaying a deposition for more than five hours.

Meanwhile, Republican Congressman Jim Jordan, the ranking member of the House Oversight Committee, wrote a letter to Reeker on Wednesday asking how and why Reeker's deposition was moved from that day to Saturday, Fox News first reported. Jordan told Reeker it would be better to hold the deposition "on a business day to allow robust Member attendance and participation."

Jordan said Republican members of the committees conducting the impeachment inquiry were "surprised and disappointed" that Reeker agreed to testify on a Saturday, and accused House Intelligence Committee Adam Schiff of scheduling the deposition on a Saturday to "further limit Member attendance and participation," according to a copy of the letter published by Axios.

Rebecca Kaplan, Stefan Becket, Arden Farhi and Kathryn Watson contributed to this report