U.S. President Donald Trump on Sunday called on the whistleblower whose concerns prompted an impeachment inquiry to come forward, appealing directly to the intelligence community employee in a stark departure from norms of conduct in such cases.

The Republican president is under increasing pressure as the House of Representatives forges ahead with its investigation of whether Trump solicited help from Ukraine as he seeks re-election next year. Leaders of the Democratic-controlled House expect to begin public hearings in the next few weeks.

The inquiry was launched after a whistleblower complaint from an intelligence official concerned that the president's actions on Ukraine were illegal and jeopardized national security.

"The Whistleblower got it sooo wrong that HE must come forward. The Fake News Media knows who he is but, being an arm of the Democrat Party, don't want to reveal him because there would be hell to pay. Reveal the Whistleblower and end the Impeachment Hoax!" Trump said in a Twitter post.

Trump has repeatedly lashed out at the whistleblower and called for his identity to be made public. He denies any impropriety and says the impeachment is politically motivated.

It was the first time he appealed directly for the whistleblower to come forward.

The impeachment focuses on Trump's request in a July 25 phone call with Ukrainian President Volodymyr Zelenskiy for an investigation of former Vice President Joe Biden, a political rival, and his son. Lawmakers are also looking at testimony that Trump withheld $391 million in U.S. aid from Ukraine to pressure Zelenskiy to launch, and publicly announce, such a probe.