Washington, Sep 26 (IANS): US lawmakers were preparing to question President Donald Trump's top intelligence official over a whistleblower complaint that has sparked an impeachment inquiry.

The development comes after Acting National Intelligence Director Joseph Maguire had initially refused to share the complaint with Congress, the BBC reported.

The complaint refers to a controversial phone call between Trump and his Ukrainian counterpart Volodymyr Zelensky, where the former pushes the latter to investigate corruption claims involving Democratic presidential hopeful Joe Biden's son, Hunter.

Biden has said that a rough transcript of the conversation released on Wednesday shows that Trump betrayed his oath of office.

Democrats on Tuesday opened an impeachment inquiry against the Trump, accusing him of seeking foreign help in the hope of smearing Biden and of using military aid to Ukraine as a potential bargaining tool.

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.

Congress's investigation focuses partly on whether Trump abused his presidential powers and sought to help his own re-election by seeking the aid of a foreign government to undermine Biden.

Trump denies putting pressure on Zelensky and dismissed the impeachment proceedings as a "hoax" and a "witch-hunt".