Trump is facing impeachment over allegations that he withheld aid from Ukraine in return for damaging info on Joe Biden's son.
U.S. House Speaker Nancy Pelosi will announce later on Tuesday that the House is launching a formal inquiry into the impeachment of President Donald Trump, the Washington Post reported.
The Post cited unnamed Democratic officials, who it said spoke on the condition of anonymity to speak frankly.
Earlier during an appearance at an event sponsored by The Atlantic magazine, Pelosi declined to say what action she planned, but told the audience, "It's absolutely, absolutely essential that the president be held accountable. No one is above the law."
A Republican, Trump on Tuesday confirmed he had withheld nearly US$400 million in U.S. aid to Ukraine but denied he did so as leverage to get Ukrainian President Volodymyr Zelenskiy to initiate an investigation that would damage Joe Biden, the front-runner for the 2020 Democratic presidential nomination.
In Washington, Pelosi was meeting with fellow Democratic lawmakers to consider impeachment of the president, who has withstood repeated scandals since taking office in January 2017.
In a Twitter post, Trump said a "complete, fully declassified and unredacted" transcript of the July 25 call would be released on Wednesday.
Trump on Sunday acknowledged that he had discussed Biden and Biden's son Hunter, who had worked for a company drilling for gas in Ukraine, with Zelenskiy.
But Trump on Monday denied trying to coerce Zelenskiy in the phone call to launch a corruption investigation into Biden and his son in return for the U.S. military aid.