US Ambassador to the European Union Gordon Sondland is enmeshed in the scandal over how President Donald Trump tried to pressure Ukraine into investigating his political rival Joe Biden

Washington (AFP) - The White House on Tuesday barred a top US diplomat from speaking to Congress as part of the impeachment investigation of Donald Trump, dramatically escalating the battle with Democrats over his presidency.

In response, Democrats heading the probe announced they will subpoena the witness, Ambassador to the European Union Gordon Sondland, for his testimony and documents.

The scandal over how Trump tried to pressure his Ukrainian counterpart into investigating his political rival Joe Biden has dominated political discourse in Washington, as Democrats weigh possible impeachable offenses by the president including obstruction of justice.

The scandal has now ensnared Sondland, who agreed to be deposed by House Democrats probing the matter, according to his attorney Robert Luskin.

But in a midnight phone message, the State Department “directed… Sondland not to appear today for his scheduled transcribed interview,” Luskin said.

Trump took to Twitter to own the decision to bar Sondland’s deposition, effectively acknowledging he was stonewalling the investigation.

“Unfortunately he would be testifying before a totally compromised kangaroo court,” Trump said, decrying that the rights of Republicans were being “taken away, and true facts are not allowed out for the public to see.”

Democrats have delivered subpoenas to multiple administration officials, including Secretary of State Mike Pompeo, ordering them to turn over relevant documents.

The White House has largely refused to cooperate.

House Speaker Nancy Pelosi formally launched the impeachment inquiry last month after revelations that Trump pressured Ukraine’s President Volodymyr Zelensky in a controversial July 25 phone call.

The top Democrat in Congress told reporters that preventing Sondland from testifying shows that “the president is obstructing Congress from getting the facts that we need.”

Biden, the former vice president who is seeking the 2020 Democratic nomination, joined in the chorus of Democratic condemnation, tweeting that Trump “must stop stonewalling Congress.”

- ‘Evidence of obstruction’ -

California Democrat Adam Schiff, chairman of the House Intelligence Committee, is leading the impeachment investigation into President Donald Trump for alleged abuse of power

Sondland is a wealthy hotelier who was a major donor to Trump’s 2016 presidential campaign.

He was one of a handful of US diplomats on a text message chain between July and September, provided to Democrats leading the probe, that go to the heart of the investigation’s focus.

The messages between the diplomats, Trump’s personal lawyer Rudy Giuliani and a Ukrainian presidential aide show that they helped coordinate administration efforts to pressure Kiev into investigating Biden as the country sought US military aid and access.

The top Democrats leading the impeachment investigation warned that the White House’s silencing of Sondland only bolsters arguments that illicit obstruction is taking place.

House Intelligence Committee chairman Adam Schiff said investigators have learned that Sondland has text messages or emails on a personal device that are “deeply relevant” to the probe.

Schiff has requested those communications, but “the State Department is withholding those messages as well,” he said.

“The failure to produce this witness, the failure to produce these documents” was “additional strong evidence of obstruction of the constitutional functions of Congress,” Schiff told reporters.

- ‘Crazy,’ ‘frightening’ call -

As the scandal deepened, new details emerged that highlight the extent of concern among White House officials immediately after Trump’s phone call with Zelensky.

US President Donald Trump says the impeachment investigation of him is a "kangaroo court"

The whistleblower who filed a complaint over the call said in a memo that a White House official who listened to the telephone discussion described it as “crazy” and “frightening,” Fox News and The New York Times reported.

The official was “visibly shaken by what had transpired” in the call, the whistleblower reportedly wrote in the memo.

Lawmakers have an opportunity on Friday to hear from another key witness in the Ukraine scandal: the former US ambassador to Kiev, Marie Yovanovitch, who is scheduled to appear before the House Intelligence Committee.

US media have reported that Trump removed her from her post because she opposed his efforts to get Ukraine to investigate Biden.

It remained unclear whether Trump will seek to block her testimony as well.

Meanwhile, Republicans who back Trump calling on Ukraine and China to launch investigations of the Bidens, under the guise of rooting out corruption, have launched a counter-offensive.

Senate Judiciary Committee chairman Lindsey Graham announced Tuesday that he will invite Giuliani to testify before the panel as part of a new Senate anti-corruption inquiry related to Ukraine.