Donald Trump said he would release a “fully declassified and unredacted transcript” of his call with the president of Ukraine just hours before Nancy Pelosi was set to deliver a major statement on investigations into the president.

In a tweet before House Democrats were set to convene and discuss launching formal impeachment proceedings into Mr Trump, the president wrote: “I am currently at the United Nations representing our Country, but have authorised the release tomorrow of the complete, fully declassified and unredacted transcript of my phone conversation with President Zelensky of Ukraine.”

“You will see it was a very friendly and totally appropriate call,” he added. “No pressure and, unlike Joe Biden and his son, NO quid pro quo! This is nothing more than a continuation of the Greatest and most Destructive Witch Hunt of all time!”

Democrats on Capitol Hill have called for the transcript's release after it was reported Mr Trump repeatedly urged Ukrainian President Volodymyr Zelensky to launch investigations into his 2020 rival's son, Hunter Biden, who previously worked a Ukrainian energy firm. There have been no findings of wrongdoing on the part of the former vice president or his son.

The phone call gained controversy after initial reports indicated a member of the US intelligence community had filed a whistle-blower report involving the president's interactions with a foreign leader. Democrats have also called for that complaint to be delivered to Congress, a move they say is included in their legal oversight responsibilities. The president and his administration have refused to release the complaint.

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House Intelligence Committee Chairman Adam Schiff, House Oversight and Reform Chairman Elijah Cummings and House Foreign Affairs Committee Chairman Eliot Engel wrote White House Counsel Pat Cipollone and threatened an “escalated” fight if documents aren’t turned over by Thursday.

A wave of Democrats meanwhile threw their support behind starting formal proceedings to remove the president from office, including Mr Biden himself, who said Congress should move to impeach Mr Trump if he failed to cooperate.

John Lewis, the Democratic civil rights icon, has also endorsed impeachment proceedings in the House after withholding his opinion on the matter for several months.

He said in a speech on the House floor on Tuesday morning that “we cannot delay” and now is the time to act.

Mr Lewis is one of the most influential Democrats in his caucus and an ally of Ms Pelosi, who has so far discouraged impeachment. He said he’s been “patient while we have tried every other path” and “the future of our democracy is at stake.”

It was not immediately clear what impact Mr Trump's tweets could have on Ms Pelosi's possible decision to support his impeachment. Reports indicated she was preparing to announce formal proceedings just before he released the statement announcing the forthcoming release of the transcripts.

The House Intelligence Chairman announced shortly after Mr Trump's tweets that the person behind the whistle-blower complaint has been in contact with the committee.

"We have been informed by the whistleblower’s counsel that their client would like to speak to our committee and has requested guidance from the Acting DNI as to how to do so," he wrote in a tweet.

"We‘re in touch with counsel and look forward to the whistleblower’s testimony as soon as this week," Mr Schiff added.

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Ms Pelosi hinted she would still move forward with announcing her apparent public support for impeachment, saying at an event on Tuesday afternoon: "We have to have the facts. That's why I've said as soon as we have the facts, we're ready. Now we have the facts, we're ready ... for later today."