President Donald Trump has branded Democrats a "disgrace" for holding impeachment proceedings while he attends a NATO summit in England and rejected participating in what he called "a hoax".

"The Democrats, the radical-left Democrats, the do-nothing Democrats, decided when I'm going to NATO - this was set up a year ago - that when I'm going to NATO, that was the exact time," Mr Trump said angrily on departing the White House.

"It's an absolute disgrace what they're doing to our country," he said. "The whole thing is a hoax. Everybody knows it."

While Mr Trump is away, a Congressional panel will begin drawing up charges against US President Donald Trump on Wednesday as the likelihood grows of his becoming only the third US leader ever impeached.

The Acting Chief of Staff Mick Mulvaney (C) has so far refused to testify in the hearings

Yesterday, Mr Trump's chief White House lawyer, Pat Cipollone, told the Democratic leader of the committee Jerry Nadler that he was rejecting an invitation to send representatives to the session.

"We cannot fairly be expected to participate... while it remains unclear whether the Judiciary Committee will afford the President a fair process through additional hearings," Mr Cipollone wrote.

But Mr Cipollone also did not rule out White House participation in the hearings entirely, instead requesting further details from Mr Nadler on how the inquiry will be conducted and saying he will give him an answer by next Friday.

Mr Trump has been unable to quash allegations in the House of Representatives investigation that he illegally sought help from Ukraine to obtain dirt on his domestic political rivals, including possible 2020 challenger Joe Biden.

Mr Nadler's committee meets on Wednesday to begin determining whether the evidence compiled in the investigative phase of the process meets the constitutional impeachment standard of "treason, bribery, or other high crimes and misdemeanors".

The committee is expected to consider at least four counts, or articles of impeachment, including abuse of power, bribery, contempt of Congress and obstruction of justice.

"The president has accepted or enlisted foreign nations to interfere in our upcoming elections," said House impeachment leader Adam Schiff, a Democrat, in a 25 November statement.

House impeachment leader Adam Schiff

"This is an urgent matter that cannot wait if we are to protect the nation's security and the integrity of our elections."

Mr Trump and Republicans continue to brand the accusations and process as a political "witch-hunt".

"This is a complete American waste of time," said Doug Collins, who as the leading Republican on the Judiciary Committee will lead that party's counterattack.

The inquiry could go fast as Democrats reportedly want to have the entire house vote on impeachment before the end of the year.

They are confident that they have strong evidence that Mr Trump committed impeachable offences, despite the White House blocking the testimony of top presidential aides and refusing to hand over subpoenaed documents.

That refusal has angered Democrats.

"We're not going to play games with them," Val Demings, a Democratic member of the Judiciary Committee, told ABC on Sunday.

"They need to respond to the request for documents and obey those lawful subpoenas."

Mr Trump is alleged to have withheld military aid for Ukraine, which was desperately sought by Kiev amid the ongoing border conflict against pro-Russian separatists, as well as a desired high-profile summit with President Volodymyr Zelensky.

Ukraine's leader Volodymyr Zelensky with US President Donald Trump

In exchange, Mr Trump wanted Ukraine to open investigations into Mr Biden and a widely discredited theory that Ukraine interfered in the 2016 US elections on behalf of the Democrats.

If the articles of impeachment are passed by the Democratic-controlled house as expected, Mr Trump would go on trial in January in the Senate for removal from office.

Given Republican control of that chamber and the high hurdle of a two-thirds majority needed to convict, the president would very likely be cleared.

Still, Democrats see the process as important in reinforcing the rule of law and pushing back against a powerful president seen as showing little respect for Congress's powers.

Mr Nadler had last week invited Mr Trump and his attorneys to take part in the first hearing on Wednesday, an invitation that Mr Cipollone declined.

Jerry Nadler

"It is too late to cure the profound procedural deficiencies that have tainted this entire inquiry," Mr Cipollone wrote in reply.

"Nevertheless, if you are serious about conducting a fair process going forward, and in order to protect the rights and privileges of the President, we may consider participating in future Judiciary Committee proceedings if you afford the Administration the ability to do so meaningfully," he added.

But Mr Nadler has said that Mr Trump's ability to participate could depend on whether he continues to refuse to make witnesses and documents available.

One Republican on the committee, Tom McClintock, told Fox that he believed top Trump aides "absolutely" should testify.

Democrats want to hear from former national security adviser John Bolton, acting White House chief of staff Mick Mulvaney and Secretary of State Mike Pompeo.

All three should have direct knowledge of Mr Trump's actions toward Ukraine.

So far they have refused to testify, claiming "absolute immunity" as confidants of the president.