Washington, DC - House Speaker Nancy Pelosi said in a letter to legislators on Friday she will send articles of impeachment against President Donald Trump to the United States Senate next week.

The procedural move will end a nearly four-week delay in which Democrats sought to raise public pressure on Senate Republicans to agree to call key witnesses and demand documents in the trial.

"The American people have clearly expressed their view that we should have a fair trial with witnesses and documents, with more than 70 percent of the public stating that the president should allow his top aides to testify," Pelosi said in the letter.

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The House of Representative impeached Trump on December 18 for abuse of power related to his dealings with Ukraine and obstruction of Congress for refusing to cooperate with the House impeachment investigation.

Democrats want four witnesses, including former National Security Adviser John Bolton and acting White House Chief of Staff Mick Mulvaney, to testify in a Senate trial.

Senate Majority Leader Mitch McConnell, a close Trump ally, announced on Tuesday Republicans would not commit to calling witnesses but rather would hear the House's case and the president's defence first.

Bolton, who was a party to Oval Office conversations with Trump about Ukraine, said in a statement on Monday he would be willing to testify to the Senate if subpoenaed.

US Speaker of the House Nancy Pelosi answers questions during a news conference at the US Capitol [File: Win McNamee/Getty Images/AFP]

Democrats have accused McConnell and Senate Republicans of attempting to orchestrate a cover-up of Trump's actions by refusing to allow witnesses.

McConnell "showed his true colours and made his intentions to stonewall a fair trial even clearer" by supporting a Senate resolution that would dismiss the articles of impeachment, Pelosi said in her letter to colleagues.

"A dismissal is a cover-up and deprives the American people of the truth," Pelosi said in her letter.

Pelosi listed a series of new disclosures of information since the House voted to impeach Trump in December, including White House and Defense Department emails obtained by outside groups and media organisations showing the president's role in the Ukraine pressure campaign.

The House will vote next week on a resolution naming managers who will serve as prosecutors of the impeachment case in the Senate and formally transmitting the articles, Pelosi's letter said.

A trial in the Senate is expected to end with an acquittal. The chamber is made up of 53 Republicans, 45 Democrats and two independents who caucus with the Democrats. At least 20 Republicans would have to vote with all Democrats and two independents to remove the president from office.

The House impeachment inquiry centred on a July 25 phone call between the US president and his Ukrainian counterpart. Democrats allege Trump abused his power of office by organising a pressure campaign to exhort a promise from Ukrainian President Volodymyr Zelenskyy to announce investigations into Biden and a debunked conspiracy theory that Ukraine, not Russia, interfered in the 2016 US elections.

On the July call, Trump urged Zelenskyy to open an investigation into Biden, who is a 2020 Democratic presidential frontrunner, and his son, Hunter, who had served on the board of a Ukrainian gas company. There has been no evidence of wrongdoing by the Bidens.

At the time of the call, the Trump administration was withholding nearly $400m in military assistance to Ukraine.

Trump has repeatedly denied any wrongdoing and labelled the impeachment a "hoax".