Vice President Pence rejected House Intelligence Committee Chairman Adam Schiff (D-Calif.)'s request Saturday to declassify more material from an aide's testimony in the ongoing House impeachment investigation, saying that his office is unsure what information the lawmaker is requesting.

"Because Adam Schiff continues to operate in an underhanded manner, the Office of the Vice President does not even know what he wants declassified," Pence spokeswoman Katie Waldman told USA Today. "It's simply further proof that this shoddy committee is pursuing a sham investigation."

In a Friday letter, Schiff's requested further information on the testimony of Jennifer Williams, a career foreign service officer and Pence staffer who testified before lawmakers last month that a conversation between President Trump and Ukrainian President Volodymyr Zelensky was "unusual, and more of a political nature."

Schiff requested in the letter that Pence declassify the contents of a Sept. 18 call he held with Zelensky.

Williams testified before about the call, but later told lawmakers that the details of the conversation were classified. Williams's attorney told the Intelligence Committee that she "reviewed certain materials that cause her [to] recall additional information" about the call, but Pence's office later informed her that the new information was classified. She later provided a classified written submission regarding the call.

"Today, the House Intelligence Committee sent the following letter to Vice President Mike Pence requesting that the Office of the Vice President, or another relevant classification authority, declassify supplemental testimony provided by Jennifer Williams to the Committee, as noted in our report," an intelligence panel aide told The Hill.

"While Ms. Williams already testified generally to the phone call in question, declassification of this supplemental testimony will allow the Congress to see further corroborative evidence as it considers articles of impeachment, and provide the public further understanding of the events in question," the official added.

In his letter, Schiff cited Pence saying last month that he had "no objection at all" to releasing further details about his call with Zelensky. The White House counsel's office has refused to release the transcript of the call, USA Today reported.

"Your willingness to release the transcript of your calls with President Zelensky necessarily means that you do not believe there is anything contained therein that would cause any harm to U.S. national security if publicly disclosed," Schiff wrote in the Friday letter.

The Hill has reached out to Pence's office for comment.