President Donald Trump accused the Democratic Party on Wednesday of wasting time on the impeachment probe sparked by the Ukraine scandal engulfing the White House, dismissing the inquiry as "bullsh*t".

"The Do Nothing Democrats should be focused on building up our country, not wasting everyone's time and energy on BULLSH*T, which is what they have been doing ever since I got overwhelmingly elected in 2016, 223-306.

"Get a better candidate this time, you'll need it!"

Trump repeatedly misstates the Electoral College vote in his 2016 presidential race against Democrat, Hillary Clinton. The official count was 304 to 227.

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 unimaginable for a president, including his claim late on Tuesday that this is "not an impeachment, it is a coup".

He amplified the message on 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 insulted 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 co-operate 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," said Trump.