House Democrats have subpoenaed the U.S. ambassador to the European Union for testimony and documents relating to their impeachment inquiry into President Trump over his July 25 phone call with the Ukrainian president, escalating a clash with the White House which just an hour before said it wouldn't comply with the "illegitimate and unconstitutional" inquiry.

The House Intelligence, Foreign Affairs and Oversight Committees asked Gordon Sondland to produce a laundry list of documents by Oct. 14 and appear at a deposition on Oct. 16, according to a news release.

Sondland's failure to comply with the subpoena "shall constitute further evidence of obstruction of the House's impeachment inquiry and may be used as an adverse inference against you and the president," the letter read.

The State Department blocked Sondland from appearing at a scheduled closed-door deposition on Tuesday. President Trump tweeted that he would "love to send Ambassador Sondland [...] but unfortunately, he would be testifying before a totally compromised kangaroo court."

House Intelligence Committee Chairman Adam Schiff, D-Calif., told reporters Tuesday morning that the move was "yet additional strong evidence of obstruction of the constitutional functions of Congress."

Schiff and other Democratic committee chairmen later released a statement saying the administration’s "interference" amounted to "obstruction of the impeachment inquiry."

Democrats have subpoenaed Secretary of State Mike Pompeo, who missed a Friday deadline to comply. They also subpoenaed the Pentagon and the White House Office of Management and Budget for Ukraine documents on Tuesday.

HOUSE DEMS BELIEVE THEY HAVE ENOUGH VOTES TO PASS ARTICLES OF IMPEACHMENT, SENIOR MEMBER SAYS

House Democrats launched their impeachment inquiry into President Trump last month after an intelligence community member's whistleblower complaint alleged he took part in a "quid pro quo" with Ukrainian President Volodymyr Zelensky during their July 25 call.

The whistleblower accused Trump of "using the power of his office to solicit interference from a foreign country in the 2020 U.S. election," claiming the president used $400 million in military aid as leverage to force officials in Ukraine to launch an investigation into 2020 presidential hopeful Joe Biden and his son Hunter, and their business dealings in Ukraine.

Trump has denied the allegations and hit back at Democrats who have supported the inquiry.

READ THE WHITE HOUSE LETTER

The White House sent an eight-page letter to House Speaker Nancy Pelosi, D-Calif., on Tuesday that read: "President Trump and his administration reject your baseless, unconstitutional efforts to overturn the democratic process. Your unprecedented actions have left the president with no choice.

"In order to fulfill his duties to the American people, the Constitution, the Executive Branch, and all future occupants of the Office of the Presidency, President Trump and his administration cannot participate in your partisan and unconstitutional inquiry under these circumstances."

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Fox News' John Roberts, Gregg Re and Adam Shaw contributed to this report.