The White House has officially refused to cooperate in the impeachment inquiry launched by Democrats targeting US President Donald Trump, saying it was "constitutionally invalid".

Key points: The White House's lawyer says the probe "violates fundamental fairness"

The White House's lawyer says the probe "violates fundamental fairness" It will not cooperate with the inquiry without a House of Representatives vote

It will not cooperate with the inquiry without a House of Representatives vote It says all previous impeachment inquiries against presidents came after a vote

The comments were made in a letter signed by White House counsel Pat Cipollone and sent to House of Representatives Speaker Nancy Pelosi, the top US Democrat, and the Democratic chairs of the House intelligence, foreign affairs and oversight committees.

The inquiry was started amid accusations from a government whistleblower that Mr Trump sought Ukraine's help in investigating Democratic rival Joe Biden.

Mr Cipollone wrote in the letter that the inquiry had progressed in a "manner that violates fundamental fairness and constitutionally mandated due process", and "lacks any legitimate constitutional foundation".

He described the inquiry as a "a naked political strategy" that "plainly seeks to reverse the election of 2016".

"Your unprecedented actions have left the President with no choice," the letter said.

"President Trump and his administration cannot participate in your partisan and unconstitutional inquiry under these circumstances."

The move came hours after the Trump administration blocked the US ambassador to the European Union from giving testimony in the impeachment investigation.

Gordon Sondland had been due to meet behind closed doors with staff of three Democratic-led House committees on Tuesday morning. He had appeared to agree voluntarily without a subpoena.

Gordon Sondland (right) had appeared willing to meet with investigators before he was blocked. ( AP: Pablo Martinez Monsivais )

Mr Cipollone said the White House would refuse to cooperate with the probe unless there was a vote of the full House of Representatives on whether an impeachment inquiry should be opened.

The letter argued the three other impeachment inquiries in American history, against presidents Andrew Johnson, Richard Nixon and Bill Clinton, all included House votes, and this should serve as precedent for the impeachment inquiry of Mr Trump.

Ms Pelosi argued the impeachment inquiry she launched was constitutional, and no House vote was necessary at this juncture.

"Proceeding without a House vote is unprecedented in the history of our nation. In every prior occasion for a presidential impeachment inquiry there has been a vote of the House," said a senior administration official, speaking as Mr Cipollone's letter was released.

The letter said Mr Trump had been denied basic due process rights, such as to cross-examine witnesses, call witnesses to testify, receive transcripts of testimony, and have access to evidence.

Ms Pelosi has said the inquiry she launched was constitutional, and a vote was not necessary. ( Reuters: Kevin Lamarque )

"All of this violates the Constitution, the rule of law, and every past precedent," the letter said.

It said the Supreme Court had recognised that due process protections applied to all congressional investigations.

The letter was the result of an intense behind-the-scenes effort in recent days by White House lawyers to respond to the Democrats' impeachment bid.

One concern the White House has involves the whistleblower — House Democrats are working to protect the identity of the person who has accused Mr Trump of impropriety in his dealings with Ukraine.

"There shouldn't be a situation where you can have a primary witness, an accuser in an impeachment inquiry, and the President never able to know who the accuser is and never able to cross-examine him," the senior administration official said.

Mr Trump has called the inquiry a partisan "witch hunt," and released a summary transcript of his July 25 phone call with Ukraine President Volodymyr Zelenskiy in a bid at damage control.

Reuters/AP