Washington, Oct 2 (AFP) President Donald Trump raged against impeachment Wednesday, tweeting that the process was "BULLSHIT" and calling for the lead investigator in Congress to be charged with treason.

Trump -- accused of leaning on Ukraine's president to dig up dirt on one of his main 2020 election rivals -- fired the swearword at opposition Democrats.

Adam Schiff, the Democrat leading the impeachment probe in the lower chamber of Congress, told reporters there is a "real sense of urgency" to press forward.

Trump fought back with language that would once have been inconceivable for a president, including his claim late Tuesday on Twitter that this is "not an impeachment, it is a COUP."

While meeting with Finnish President Sauli Niinisto in the White House, Trump said Schiff was "a low life" who should be arrested for "treason."

Trump insists that he did nothing wrong in a phone call with Ukrainian leader Volodymyr Zelensky and on Wednesday got support from Russia's President Vladimir Putin, who said he saw "nothing compromising" in the conversation.

Given Trump's controversial history with Putin, it was unlikely that the Kremlin leader's backing would do much to calm waters back in Washington.

After keeping pronouncements mostly to Twitter over the last week, Trump used his press appearance with Niinisto to let rip in front of the cameras, calling his treatment "a disgrace."

Trump is accused of having pressured Zelensky to help him by opening a corruption investigation against leading Democratic presidential candidate Joe Biden in a July 25 phone call.

He is alleged to have suggested that military equipment Ukraine sought to beef up its defenses against Russia would be contingent on him getting that favor.

A whistleblower, so far only identified as someone from the intelligence services, went to the authorities with concerns about the phone call, triggering the impeachment inquiry.

Trump has likened the whistleblower to a spy and called for his or her identity to be made public, although by law whistleblowers are protected. He has also retweeted a warning that his removal from office could trigger "civil war."

Schiff on Wednesday called Trump's comments about the whistleblower a "blatant effort to intimidate witnesses."

The State Department's inspector general was due to brief congressional committees Wednesday on what it said were documents "related to the State Department and Ukraine."

It was not clear what that would entail, but the State Department is closely caught up in the probe, with Secretary of State Mike Pompeo confirming Wednesday previous reports that he listened in during the Zelensky call.

Pompeo had earlier accused Democrats of trying to "intimidate" and "bully" State Department employees. Democrats said he was "stonewalling" their investigation.

Pompeo and Trump's controversial personal lawyer Rudy Giuliani have been subpoenaed to provide documents. Five diplomats have so far been summoned to testify.

Pompeo suggested that the committees could be forced to subpoena the five officials, and that the State Department and White House could seek to limit what they can talk about.

"I will use all means at my disposal to prevent and expose any attempts to intimidate the dedicated professionals whom I am proud to lead," Pompeo said.

News reports said the State Department's former special envoy to Ukraine, Kurt Volker, would testify Thursday and that the ex-ambassador to Kiev, Marie Yovanovitch, would appear behind closed doors on October 11.

Volker had been sought by Giuliani to help pressure Zelensky, while Yovanovitch was removed earlier this year as ambassador after she reportedly resisted that effort.

Pompeo himself risks greater pressure after the Democratic heads of the three investigating congressional committees said his being in on the phone call made him "a fact witness."

The Democrats say that subpoenas could also be issued to force White House compliance with their demands for documentation.

"We're not fooling around here," Schiff said.

"We don't want this to drag on months and months and months, which appears to be the administration's strategy." (AFP)

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