A defiant White House has refused to cooperate with the Democrats in the impeachment inquiry against President Donald Trump, saying their move was "baseless" and "constitutionally invalid".

The inquiry is trying to find out if President Trump held back aid to Ukraine to push it to investigate former US vice president Joe Biden, who is running for the 2020 Democratic presidential nomination.

Speaking to reporters, White House Press Secretary Stephanie Grisham said that the impeachment probe against President Trump is purely political and an attempt to overturn the democratic process.

"The President has done nothing wrong and the Democrats know it. For purely political reasons, the Democrats have decided their desire to overturn the outcome of the 2016 election allows them to conduct a so-called impeachment inquiry that ignores the fundamental rights guaranteed to every American," she said.

"These partisan proceedings are an affront to the Constitution - as they are being held behind closed doors and deny the President the right to call witnesses, to cross-examine witnesses, to have access to evidence, and many other basic rights," she said soon after the White House sent a letter to the Opposition Democratic leaders in the House of Representatives, informing them of the Trump administration's decision.

The Democrats are examining whether there are grounds to impeach Trump, a Republican, based on a whistle-blower's account that said he asked Ukrainian President Volodymyr Zelenskiy in a July 25 phone call to help investigate his Democratic political rival Biden.

In the 8-page letter, White House counsel Pat Cipollone rejected the allegations against President Trump, terming it baseless and unconstitutional.

"President Trump and his Administration reject your baseless, unconstitutional efforts to overturn the democratic process, he said in his letter dated October 8, a copy of which was released to the press.

The White House letter comes hours after the Trump administration blocked the US ambassador to the European Union Gordon Sondland from appearing before a congressional impeachment investigation.

"Your unprecedented actions have left the President with no choice. In order to fulfil his duties to the American people, the Constitution, the Executive Branch, and all future occupants of the Office of the Presidency, President Trump and his Administration cannot participate in your partisan and unconstitutional inquiry under these circumstances," Cipollone said.

He said Trump is focused on fulfilling his promises to the American people.

Reacting to the letter, House of Representatives Speaker Nancy Pelosi said President Trump was "trying to make lawlessness a virtue".

"For a while, the President has tried to normalise lawlessness. Now, he is trying to make lawlessness a virtue, Pelosi said in a statement.

"The American people have already heard the President's own words - do us a favour though. The President's actions threaten our national security, violate our Constitution and undermine the integrity of our elections. The White House letter is only the latest attempt to cover up his betrayal of our democracy, and to insist that the President is above the law, she said.

Later a senior administration official told reporters that for the first time in America's history, the House of Representatives is now purporting to proceed on an impeachment inquiry against the President without conducting a vote on the House floor.

"This has never happened in the history of the country. There's always been, in every prior inquiry into a presidential impeachment, a vote on the House floor," the official asserted.

The House is purporting to proceed against the president simply on the basis of a news conference held by Pelosi, the official said.

"And that is simply not sufficient to establish the sort of accountability to show the will of the House to embark on this that is required out of due respect for the separation of powers and for the vote of the American people that was taken in the last election, said the official, who spoke with reporters on condition of anonymity.

The official said even more importantly, the second legal and constitutional flaw is that the House is purporting to proceed without providing any of the due process protections that have been provided in all modern presidential impeachment inquiries in the past.

"They have denied the President the right to cross-examine witnesses, to call witnesses, to receive transcripts as testimony, to have access to evidence, to have counsel present -- all of these. And these have been in the 1970s, in the inquiries into President Richard Nixon; in the late 1990s, the inquiries into impeachment with President Bill Clinton. All these rights were available. It is unprecedented that they are being denied here, and it violates basic due process standards, the official said.

In addition to that, by failing to have any resolution on the House floor, the House has avoided giving even the minority on the committees the right to subpoena.