WASHINGTON - Democrats in Congress, answering Republican complaints that their impeachment investigation of US President Donald Trump is being conducted in secret, plan a vote on Thursday on how to make their inquiry public, a significant new stage in the probe.

Trump and his fellow Republicans have for weeks branded the work of committees probing Trump's overtures to Ukraine as illegitimate, arguing the full Democratic-led House of Representatives had failed to authorize their investigations in a public vote.

Meeting behind closed doors, the House Intelligence, Foreign Affairs and Oversight committees have been looking into the possibility that Trump violated federal law by seeking foreign help for his November 2020 re-election efforts.

House Rules Committee Chairman James McGovern said on Monday: "I will be introducing a resolution to ensure transparency and provide a clear path forward."

House Intelligence Committee Chairman Adam Schiff said in a statement the legislation would establish a format for open hearings.

The U.S. Constitution gives the House broad authority to set ground rules for an impeachment inquiry and Democrats say they are following House rules on investigations.

Several administration officials, including a former deputy national security adviser on Monday, have failed to testify to House committees engaged in the probe.

House Speaker Nancy Pelosi said in a letter to her fellow House Democrats the House will vote this week - on Thursday, according to a senior Democratic aide - on a resolution that spells out how future public hearings will be held.

Pelosi promised to provide legal protections for Trump.

A source familiar with the probe said the public hearings will be held by the Intelligence Committee and that the transcripts from closed depositions with witnesses will be made public. A senior House Democratic aide said the hearings could begin within the next month.

The measure will set the stage for House investigating committees to forward evidence they have collected to the House Judiciary Committee, which would then decide whether to advance articles of impeachment against Trump.

Even if the House impeaches Trump, he would face a trial in the Republican-controlled Senate, which for the moment seems unlikely to convict the president and force his ouster.

Impeachment requires a simple majority in the 435-member House but conviction demands the support of a two-thirds majority in the 100-member Senate.

Charles Kupperman, a former deputy to ousted national security adviser John Bolton, failed to appear earlier on Monday before the three House panels conducting the Ukraine phase of the Democratic-led impeachment inquiry, lawmakers said.

Other panels are probing separate issues, including Trump's finances and abuses of White House security clearances.

Kupperman put off testifying while asking a court to rule on whether he should comply with a congressional subpoena or honor the Trump administration's order not to testify, his lawyers said last week.

Democrats in turn said they would not let further legal maneuverings delay their work. -Reuters