Additional current and former aides are expected to appear before the House in the coming days as part of its impeachment probe into President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's dealings with Ukraine.

Philip Reeker, acting assistant secretary of State for European and Eurasian affairs, is expected to appear in a closed session Saturday and Charles Kupperman, former deputy assistant to the president for national security affairs, is expected to appear in a closed session Monday, according to an official who's working on the impeachment inquiry.

The official said that Timothy Morrison, special assistant to the president and national security council senior director for Europe and Russia, is expected to appear in closed session on Thursday.

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Morrison was mentioned during diplomat William Taylor's testimony as someone who may be able to corroborate his assertions.

Taylor specifically described a phone call with Morrison in which Morrison allegedly said that Gordon Sondland, ambassador to the European Union, discussed a quid pro quo with Andriy Yermak, a representative for Ukrainian President Volodymyr Zelensky.

“During this same phone call I had with Mr. Morrison, he went on to describe a conversation Ambassador Sondland had with Mr. Yermak,” Taylor testified Tuesday. “Ambassador Sondland told Mr. Yermak that the security assistance money would not come until President Zelensky committed to pursue the Burisma investigation."

Reeker and Morrison were both previously slated to testify this week, but the dates of some depositions were moved to accommodate events honoring the late House Oversight and Reform Committee Chairman Elijah Cummings Elijah Eugene CummingsBlack GOP candidate accuses Behar of wearing black face in heated interview Overnight Health Care: US won't join global coronavirus vaccine initiative | Federal panel lays out initial priorities for COVID-19 vaccine distribution | NIH panel: 'Insufficient data' to show treatment touted by Trump works House Oversight Democrats to subpoena AbbVie in drug pricing probe MORE (D-Md.), who died Oct. 17.

Trump has denied wrongdoing and maintained that there was no quid pro quo involving withheld military aid.

Olivia Beavers contributed.