Testimony by two of the "three amigos" charged with running Ukraine policy for President Donald Trump will be made public on Tuesday as Democratic-led congressional committees release more transcripts from their impeachment investigation.

Rep. Adam Schiff, chairman of the House of Representatives Intelligence Committee, told reporters on Monday transcripts would be released of interviews with Kurt Volker, Trump's former special representative for Ukraine negotiations, and Gordon Sondland, the U.S. ambassador to the European Union.

Witnesses have testified that Volker and Sondland, with Trump's secretary of energy, Rick Perry, were known as the "three amigos," responsible for Trump's unofficial channel to Ukrainian government officials.