US Secretary of State Mike Pompeo on Tuesday accused Democrats of intimidation in their impeachment inquiry into President Donald Trump and refused to let State Department employees comply with orders to appear in coming days.

In a letter to Congress, Pompeo said the subpoenas “can only be understood as an attempt to intimidate, bully and treat improperly the distinguished professionals of the Department of State.” “Let me be clear: I will not tolerate such tactics, and I will use any means at my disposal to prevent and expose any attempts to intimidate the dedicated professionals whom I am proud to lead and serve alongside,” he wrote.

Citing “profound procedural and legal deficiencies” in the subpoenas, Pompeo said that depositions of the five serving or former officials ordered to begin on Wednesday “are not feasible,” without saying if they would appear on different dates.

Democrats in charge of the House are investigating whether Trump improperly pressured the new president of Ukraine to dig up dirt that could harm Democratic candidate Joe Biden.

In a subpoena issued on Friday, the House Foreign Affairs Committee ordered five officials to give depositions between October 2 and October 10 on the affair.

One of them, Kurt Volker, who was the US special representative on Ukraine, resigned on Friday.

A whistleblower complaint had said that Volker, a former US ambassador to NATO who is well respected in Washington, had helped Ukrainian officials seeking to “navigate” the unusual request from Trump.

Pompeo in his reply criticised the committee for telling the officials that a failure to appear would “constitute evidence of obstruction.” “There is no legal basis for such a threat,” he said.

He said that the officials were each being asked to produce a “vast” number of documents, which the State Department needed to review to see if they could be publicly disclosed.

Pompeo, who released the letter just as he began a visit to Italy, is a stalwart defender of Trump.

A Harvard-educated lawyer, Pompeo built his name as a congressman probing former secretary of state Hillary Clinton over a deadly attack on the US consulate in Benghazi, Libya.