US President Donald Trump has unleashed furious attacks on the impeachment inquiry launched against him by Democrats.

It comes amid an intensifying standoff between the president and Congress.

President Trump, who is 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.

On Twitter he said Democrats should be "focused on building up our Country, not wasting everyone's time and energy on BULLSHIT, which is what they have been doing ever since I got overwhelmingly elected in 2016."

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.

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

He amplified that message today standing next to Finnish President Sauli Niinisto in the White House, branding the impeachment process - announced by House Speaker Nancy Pelosi last week - as "the greatest hoax".

Donald Trump aimed his wrath directly at Mr Schiff, deeming the House Intelligence Committee chairman "a low life" who should be arrested for "treason".

But at the same time President 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.

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

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

Congress pushes back

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

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

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

President 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".

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

The intelligence chairman urged Mr Trump and the White House to treat the pending subpoena with the utmost gravity.

"We're not fooling around here," Mr Schiff said, adding that efforts to stonewall the collection of related data will be considered "evidence of obstruction of justice".

Meanwhile, the State Department's inspector general met behind closed doors with a bipartisan group of staffers from House and Senate committees.

Secretary of State Mike Pompeo, as well as Mr Trump's personal lawyer Rudy Giuliani have been subpoenaed to provide documents. Five diplomats have so far been summoned to testify.