WASHINGTON (DPA) - US President Donald Trump said on Saturday he "probably" will release a transcript of a second phone call he had with Ukrainian President Volodymyr Zelenskiy next week.

"They want to have a transcript of the other call, the second call, and I'm willing to provide that," Trump said.

The call took place in April shortly after Zelenskiy won a landslide victory in Ukraine's presidential election.

Trump previously said he would be willing to release a transcript of the call but did not specify when.

Trump's comment about the transcript come as the impeachment probe enters a new phase.

Democrats said earlier this week they will hold the first open hearings in Congress on Wednesday.

Hearings thus far have been behind closed doors.

The impeachment probe has centred on a July phone call Trump had with Zelenskiy in which he asked for a "favour" in investigating Joe Biden, a leading Democrat seeking the party's nomination to run against Trump next year.

Diplomats who listened in on the call have expressed alarm that Trump was seeking help from Ukraine with his domestic political campaign at the expense of US foreign policy objectives.

The interim ambassador to Ukraine testified this week in a closed-door session, making clear that he believed the Ukrainians were being pressed to pursue the investigation into Biden if they wanted military aid and a White House meeting, according to a transcript of his testimony.

Democrats say the president should be impeached because his actions amount to abuse of power.

Trump has called the impeachment process a "witch hunt" and part of an ongoing effort to get him out of office.