US President Donald Trump (C) rails on the Federal Reserve's monetary policy in a speech delivered at the Economic Club of New York in New York, the United States, Nov. 12, 2019. (Xinhua/Wang Ying)

Democrats responded skeptically on Monday to US President Donald Trump's declaration that he might be willing to testify in his impeachment inquiry and also said they were examining the truthfulness of his testimony in the Russia 2016 election interference probe.

Trump said on Twitter that he would "strongly consider" House of Representatives Speaker Nancy Pelosi's suggestion that he testify in the House impeachment inquiry into whether he pressured Ukraine to investigate domestic political opponent Joe Biden.

"President Trump should testify," Representative Don Beyer said in a tweet. "He should allow top White House aides to testify... He should turn over the documents Congress subpoenaed. He should end his illegal coverup."

Trump has come under increasing pressure as public hearings have started in the impeachment probe, and the Democratic-led committees conducting the investigation have released transcripts of interviews conducted in closed sessions that contain ­potentially damaging allegations against him.

Late on Monday, they released a transcript of an interview with David Holmes, a top aide from the US Embassy in Kiev, who will testify at a public hearing on Thursday.

Holmes testified that on July 26 he overheard a call, which was held on the outdoor terrace of a Kiev restaurant, between Gordon Sondland, US Ambassador to the European Union, and Trump.

In the call, he testified, Trump asked Sondland whether Ukrainian President Volodymyr Zelensky had agreed to announce an investigation into the son of former US vice president Biden, a rival in Trump's race for reelection next year.

Holmes, an aide to acting US Ambassador to Ukraine William Taylor, said he could hear the conversation and believes two others at the table could as well.

"So he's gonna do the investigation?" Holmes said he heard Trump ask.

"He's gonna do it," Sondland said, adding that Zelensky would do "anything you ask him to."

The hearings could clear the way for the House to approve articles of impeachment - formal charges - against Trump. That would lead to a trial in the Senate on whether to convict Trump and remove him from office. Republicans control the Senate and have shown little support for Trump's removal.