Washington (CNN) Assistant Secretary of Energy Melissa Burnison told the three committees involved in the House impeachment inquiry that the Energy Department is "unable to comply with your request for documents and communications at this time."

In a letter dated Friday, Burnison argues about the validity of the inquiry and contends the request is for confidential communications "that are potentially protected by executive privilege and would require careful review."

Burnison concludes by saying the department "remains committed to working with Congress."

Outgoing Energy Secretary Rick Perry 's role in the Ukraine scandal has come under scrutiny in recent weeks, and his contact with Ukrainian officials was the subject of the subpoena from House Democrats

Acting White House chief of staff Mick Mulvaney confirmed in a rare press conference Thursday that President Donald Trump had instructed Perry, at a May 23 meeting in the Oval Office, to work through his personal attorney Rudy Giuliani on Ukraine-related issues.

Multiple witnesses in the impeachment probe have testified that Giuliani meddled in foreign policy related to Ukraine, circumventing the diplomatic process in some cases.

House Democrats had also sought documents related to Perry's trip to Ukraine for President Volodymyr Zelensky's inauguration, when Perry led the US delegation in place of Vice President Mike Pence at Trump's instruction.

The rejection is the latest instance where the Trump administration has flouted House subpoenas in the Democratic impeachment inquiry.

The White House charged in a letter to House Speaker Nancy Pelosi last week that its impeachment inquiry was "illegitimate" and "unconstitutional," and that it would not participate in the investigation.

The State Department, Pentagon and Office of Management and Budget have all already failed to respond to House subpoena deadlines after Democrats demanded a host of documents related to the freezing of US security aid to Ukraine, the push for Ukraine to open an investigation and the ouster of former US Ambassador to Ukraine Marie Yovanovitch.

"The Department is unable to comply with your request for documents at this time," Robert Hood, Assistant Secretary of Defense for Legislative Affairs, wrote to the three committees leading the impeachment probe on Tuesday. "The Department understands the significance of your request for information and has taken steps to identify, preserve, and collect potentially responsive documents."

Democrats have had more success using subpoenas to bring in current and former administration officials, despite White House direction that they do not appear. This past week, former White House Russia adviser Fiona Hill, State Department Deputy Assistant Secretary George Kent and US Ambassador to the European Union Gordon Sondland all testified under subpoena.

But the Trump administration's stonewalling of the investigation has hampered the Democratic investigation in a key way: those officials did not provide documents to the committee, saying they were the possession of the Trump administration.

"We know from the additional witnesses who have come forward that there are additional documents that they have provided the State Department but have not been given to Congress," House Intelligence Chairman Adam Schiff, a Democrat from California, said this week.

While Democrats have sued in other investigations in order to try to obtain the documents the Trump administration has refused to hand over, this time they're signaling they will go another route: that defiance of subpoenas is evidence of obstruction of Congress.

"We will consider defiance of subpoenas as evidence of the President's effort to obstruct the impeachment inquiry, and we may also use that obstruction as additional evidence of the wrongfulness of the President's underlying misconduct," Schiff wrote in a letter to colleagues this week.

Republicans have defended the Trump administration's decision not to cooperate with the investigation, arguing that Schiff is not running a fair inquiry.

"I think the (Republican House) conference is totally united in the idea that we stand behind the President like the American people do," said Ohio Rep. Jim Jordan, the top Republican member on the House Oversight Committee. "This investigation, as I just said earlier, is unfair and partisan and we see it for what it is."

More subpoenas are likely to be sent from Capitol Hill to the Executive Branch as the impeachment probe marches on and more witnesses are interviewed. While the Trump administration has rejected subpoenas for documents, it still remains to be seen whether top officials that Democrats are likely to eventually seek testimony from —Mulvaney, Perry and former national security adviser John Bolton among them — will appear if they receive subpoenas.

This story has been updated.