In a surprise move regarding his impeachment proceedings, US President Donald Trump has said that he might release the transcript of the conversation over a phone call between him and Ukrainian President Volodymyr Zelenskiy.

Earlier, Trump had said over the weekend that he might release the call transcript on Tuesday, but on Monday tweeted again to notify that he would likely release the same "before week's end". The development comes a day ahead of the US Congress' impeachment inquiry against President Trump going public.

"No Due Process Scam!" the US President wrote in a tweet, continuing his defence against the prove that the Democrats are conducting as the first public hearings in the impeachment inquiry are set to begin on Wednesday.

Democrats are investigating whether there are grounds to impeach Trump over his July 25 request, in a phone call to Zelenskiy, that the latter investigate a domestic political rival, former Vice President Joe Biden.

That call prompted a whistleblower complaint that led Democrats to launch the probe in September into whether Trump abused his power by withholding nearly $400 million in security assistance to Ukraine to pressure the vulnerable US ally.

The transcript Trump said he would release is from a call with Zelenskiy on April 12, after the Ukrainian was elected president but before he took office. The White House did not release a readout of that conversation when it took place.

The investigation, formally launched six weeks ago by Democratic Speaker of the House of Representatives Nancy Pelosi, has shadowed Trump`s presidency with the threat that he could be removed from office even as he seeks re-election next year.

He is the fourth US president to face impeachment. None were removed from office, although Richard Nixon resigned as he faced almost certain impeachment in 1974 over the Watergate scandal.