U.S. Secretary of State Mike Pompeo clashed with senior Democrats on Tuesday, accusing House committee chairs of bullying and intimidation as two key former officials agreed to cooperate with the impeachment inquiry into U.S. President Donald Trump.

CNN later reported that the State Department's inspector general had requested an urgent briefing with senior congressional staff members in what an aide called a "highly unusual" move.

Trump is accused of asking Ukrainian President Volodymyr Zelenskiy to investigate political rival Joe Biden while holding millions of dollars of military aid as leverage in a call that Pompeo is reported to have listened in on. A whistleblower also claims the White House tried to cover up the incident.

Key Ukraine envoys to testify

Pompeo indicated on Tuesday that he would seek to prevent five former and present State Department officials from cooperating with the inquiry, but Reuters later reported that former U.S. ambassador to Ukraine Marie Yovanovitch and former U.S. special representative for Ukraine Kurt Volker have agreed to give testimony to committees.

Yovanovitch will give a deposition on October 11, a committee official told Reuters.

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Volker, who resigned last week after being named in the whistleblower complaint that sparked the impeachment inquiry, is expected to appear before the panel on Thursday.

There was no immediate word on whether three other officials had agreed to appear before the committees. They are Deputy Assistant Secretary of State George Kent, State Department Counselor T. Ulrich Brechbuhl and U.S. Ambassador to the European Union Gordon Sondland.

'Bullying and intimidation'

Earlier, while in a three-day trip in Italy, Pompeo pushed back on House Democrats' efforts to get the depositions from state department officials, accusing the lawmakers of bullying and intimidation.

The officials "may not attend any interview or deposition" without executive branch counsel present to control disclosure of confidential information, Pompeo wrote in a letter to House Foreign Affairs Committee Chairman Eliot Engel.

In a letter posted on Twitter, Pompeo told Engel: "I am concerned with aspects of your request that can be understood only as an attempt to intimidate, bully, and treat improperly the distinguished professionals of the Department of State, including several career Foreign Service Officers, whom the committee is now targeting.”

Engel and two other Democratic committee chairmen issued a statement accusing Pompeo of "stonewalling" the impeachment inquiry, and called him a "fact witness" in the investigation, based on media reports that he had listened in on Trump's call with Zelenskiy.

(With input from Reuters)