Secretary of State Mike Pompeo promised to prevent State Department officials with knowledge of the President’s pressure campaign against Ukraine from testifying before the House, according to a Tuesday letter from the secretary.

Pompeo branded the attempt from the House Foreign Affairs, Intelligence, and Oversight Committees to depose five State Department officials as a bid to “intimidate, bully, and treat improperly.” The secretary of state wrote in the letter to Rep. Eliot Engel (D-NY) that the planned interviews were not “feasible.”

Pompeo said that he intends to “use all means at my disposal to prevent and expose any attempts to intimidate the dedicated professionals whom I am proud to lead and serve alongside at the Department of State.”

He added that the State Department “would be in further contact with the Committee in the near future as we obtain further clarity on these matters.”

Former U.S. ambassador to Ukraine Marie Yovanovitch is the first official set to be deposed on Wednesday. After her, the recently resigned U.S. special envoy to Ukraine Kurt Volker is scheduled for a Thursday deposition, while other State Department officials — including U.S. Ambassador to the EU Gordon Sondland and Counselor T. Ulrich Brechbuhl — are scheduled for depositions next week.

Pomepo expressed the most alarm in the letter at the prospect of State Department employees appearing without a representative from the executive branch in the room.

In the past, White House lawyers have intervened in House depositions to block testimony of those called to appear.

“This amounts to an attempt to circumvent the Executive Branch’s unquestionably legitimate constitutional interest in protecting potentially privileged information related to the conduct of diplomatic relations,” Pompeo wrote, adding that the five officials set to be deposed “may not attend” without an attorney from the executive branch.

The three House panels hit back at Pompeo, saying that he was likely a “fact witness” in Congress’s inquiry.

“He should immediately cease intimidating Department witnesses in order to protect himself and the President,” the statement reads. “Any effort to intimidate witnesses or prevent them from talking with Congress—including State Department employees—is illegal and will constitute evidence of obstruction of the impeachment inquiry.”

This story has been updated.