US lawmakers are preparing to question President Trump's top intelligence official over a whistleblower complaint that sparked an impeachment inquiry, reports BBC.
Acting national intelligence director Joseph Maguire initially refused to share the complaint with Congress.
It refers to a controversial phone call between Mr Trump and the Ukrainian president, US media report.
In the call, Mr Trump pushes Volodymyr Zelensky to investigate corruption claims involving the son of Joe Biden.
Mr Biden - the leading Democratic presidential hopeful - says a rough transcript of the conversation released on Wednesday shows the president betrayed his oath of office.
Democrats opened an impeachment inquiry against the president on Tuesday, accusing Mr Trump of seeking foreign help in the hope of smearing Mr Biden and of using military aid to Ukraine as a potential bargaining tool.
President Trump has dismissed the impeachment proceedings as a "hoax" and a "witch-hunt".
The whistleblower's formal complaint was still classified on Wednesday, but some US politicians had been allowed to see it, and senior Democrats called for its immediate release.
A House of Representatives vote to impeach the president could trigger a trial in the Senate on whether to remove Mr Trump from office.
Congress's investigation focuses partly on whether Mr Trump abused his presidential powers and sought to help his own re-election by seeking the aid of a foreign government to undermine Mr Biden.
Mr Trump denies putting pressure on Mr Zelensky.