A bellicose Donald Trump unleashed furious attacks Wednesday on the impeachment inquiry launched against him by Democrats, amid an intensifying standoff between the president and Congress.

Trump – accused of leaning on Ukraine's president to dig up dirt on one of his main 2020 election rivals – resorted to coarse language in his broadsides against the investigation and his adversaries conducting it.

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Democrats should be "focused on building up our Country, not wasting everyone's time and energy on BULL****, which is what they have been doing ever since I got overwhelmingly elected in 2016," Trump tweeted.

Adam Schiff, the impeachment probe's Democratic point man in the House of Representatives, told reporters there is a "real sense of urgency" to press forward.

Trump has fought back in terms once inconceivable for a president, including his claim late Tuesday that this is "not an impeachment, it is a COUP."

He amplified the message Wednesday standing alongside Finnish President Sauli Niinisto in the White House, branding the impeachment process – announced by House Speaker Nancy Pelosi last week – as a "hoax."

Trump assailed Schiff, declaring the House Intelligence Committee chairman "a low life" who should be arrested for "treason."

But at the same time Trump acknowledged he may yet cooperate with the latest move by Democrats, who threatened to subpoena the White House for documents related to the president's efforts to get Ukraine to probe a political rival.

"We'll work together with 'shifty' Schiff and Pelosi and all of them and we'll see what happens," he said.

Trump insists he did nothing wrong in a July 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.

A White House summary of the call showed Trump pressed Zelensky several times to open a corruption investigation against leading Democratic presidential candidate Joe Biden and son Hunter, who did business in Ukraine when his father was vice president.

Neither Biden has been credibly accused of any wrongdoing.

"Biden and his son are stone-cold crooked," Trump said Wednesday.

In response, Joe Biden vowed the Republican president is "not going to destroy me" hours after Trump's remarks.

"Let me make something clear to Trump and his hatchet men and the special interests funding his attacks against me," Biden said in prepared remarks distributed by his campaign in advance of an appearance in Reno, Nevada, on Wednesday night.

"I'm not going anywhere. You're not going to destroy me. And you're not going to destroy my family. I don't care how much money you spend or how dirty the attacks get," he said.

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

Five diplomats have so far been summoned to testify.

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The State Department's former special envoy to Ukraine, Kurt Volker, will testify Thursday and Marie Yovanovitch, an ex-ambassador to Kiev, is due to appear behind closed doors on October 11, according to Schiff.

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

(Cover photo: U.S. President Donald Trump responds to a question during a joint news conference with Finland's President Sauli Niinisto at the White House in Washington, October 2, 2019.)