U.S. President Donald Trump is criticizing an impeachment inquiry against him as a "coup," while the heads of several House of Representatives committees accuse Secretary of State Mike Pompeo of blocking their efforts to gather documents and interview witnesses.

"As I learn more and more each day, I am coming to the conclusion that what is taking place is not an impeachment, it is a COUP, intended to take away the Power of the People, their VOTE, their freedoms, their Second Amendment, Religion, Military, Border Wall, and their God-given rights as a Citizen of The United States of America!" Trump wrote on Twitter.

As I learn more and more each day, I am coming to the conclusion that what is taking place is not an impeachment, it is a COUP, intended to take away the Power of the.... — Donald J. Trump (@realDonaldTrump) October 1, 2019

Majority Democrats in the House are pursuing the impeachment inquiry to see whether they want to officially bring charges against Trump under their constitutional authority to seek to remove officials who engage in "treason, bribery, or other high crimes and misdemeanors."

The State Department's inspector general is expected to meet Wednesday with staff from the House and Senate appropriations, oversight, foreign affairs and intelligence committees to discuss documents that lawmakers have requested as they probe a July phone call between Trump and Ukrainian President Volodymyr Zelenskiy.

The House intelligence, oversight and foreign affairs committees had asked to hear testimony Wednesday from former U.S. Ambassador to Ukraine Marie Yovanovitch, but that session was postponed until next week. Former U.S. envoy to Ukraine Kurt Volker is expected to speak to the committees on Thursday.

Trump has said he did nothing wrong in his discussions with Zelenskiy. A whistleblower filed a complaint expressing concern that Trump was seeking foreign interference in the 2020 election by asking Ukraine to investigate Democratic candidate Joe Biden.

Pompeo sent a letter Tuesday to House Foreign Affairs Committee Chairman Eliot Engel saying requests for State Department documents and depositions with current and former officials "can be understood only as an attempt to intimidate, bully, and treat improperly" the department's staff.

He said the requests raise "significant legal and procedural concerns," and dismissed warnings that not cooperating would amount to obstruction.

Engel, along with Oversight Committee Chairman Elijah Cummings and Intelligence Committee Chairman Adam Schiff, responded by pointing to reports that Pompeo was on Trump's call with Zelenskiy, saying that means he has an "obvious conflict of interest" and "should not be making any decisions regarding witness testimony or document production in order to protect himself or the President."

They wrote that if it is true Pompeo participated in the call, then he is "now a fact witness in the impeachment inquiry."