At the time of the call, Trump was withholding $391m in military aid from Ukraine, and conditioned a White House meeting on the probes, according to witnesses. Trump denies any wrongdoing.

The inquiry centred on a call between Trump and Ukraine's president in which Trump asks for a probe into the Bidens. Trump also wanted a probe into a conspiracy theory about the 2016 elections.

US House Speaker Nancy Pelosi said on Thursday that she will send articles of impeachment against President Donald Trump when she is ready, but it will likely be "soon".

"I'm not holding them indefinitely," Pelosi said, referring to the two articles passed by the House in December. "I'll send them over when I'm ready, and that will probably be soon."

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It has been more than three weeks since the Democratic-controlled House of Representatives voted to impeach Trump for obstruction of Congress and abuse of power related to his dealings with Ukraine.

Since then, Pelosi has effectively blocked the Senate from moving forward with a trial by delaying the transmission of the impeachment articles.

Trump was impeached in mid-December [File: Kevin Lamarque/Reuters]

Repeating her view since Trump was impeached on December 18, Pelosi said Democrats need to "see the arena in which we are sending our managers". She was referring to the House managers who will act as prosecutors in a trial.

"Is that too much to ask?" she said.

'No haggling'

A sticking point between Democrats and Senate Majority Leader Mitch McConnell is the question of witnesses. Democrats want the trial to include at least four witnesses, including acting White House Chief of Staff Mick Mulvaney and former National Security Adviser John Bolton. McConnell has indicated he wants a swift trial without witnesses.

The standoff deepened this week when McConnell announced he had the backing of enough Republicans to begin a trial as soon as the Senate received the articles of impeachment.

There will be "no haggling" on the details of the trial, McConnel said.

Under that plan, the question of witnesses would be addressed after opening arguments.

Senate Majority Leader Mitch McConnell speaks with reporters after walking off the Senate floor [File: Patrick Semansky/AP Photo]

Speaking from the Senate floor on Thursday, McConnell said the Senate will move forward with its legislative agenda unless the House transmits the articles.

"There's real business for the American people that the United States Senate needs to complete. If the speaker continues to refuse to take her own accusations to trial, the Senate will move forward next week with the business of our people," McConnell said. Politico reported that McConnell has told Republicans he expects a trial to begin next week.

For his part, Trump railed against Pelosi in a series of tweets on Thursday morning.

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

Trump is only the third president in US history to be impeached. Democrats say he abused his power of office by pressuring Ukraine to open investigations into former Vice President Joe Biden, and his son, Hunter, who had served on the board of a Ukrainian gas company, and a debunked conspiracy theory that Ukraine, not Russia, meddled in the 2016 presidential elections.

The second article of impeachment - obstruction of Congress - stems from Trump's refusal to take part in the inquiry and his instructions to top administration officials that they do the same.