If Donald Trump pressured his Ukrainian counterpart to investigate former US vice president Joe Biden then impeachment would be the only option, the US House Intelligence Committee chairman has warned.

Adam Schiff's comments came as the scandal surrounding an alleged complaint by a whistleblower rumbles on.

The complaint, which Mr Trump's administration has refused to let Congress see, is believed to relate to the Republican president's 25 July phone call with Ukrainian leader Volodymyr Zelenskiy.

Image: Adam Schiff has suggested Mr Trump may have to be impeached

US media reports claim Mr Trump repeatedly asked Mr Zelenskiy to investigate unsubstantiated charges that Mr Biden, when he was vice president, threatened to withhold US aid unless a prosecutor who was looking into a gas company in which his son was involved was fired.

Democrat Mr Schiff told CNN on Sunday: "If the president is essentially withholding military aid at the same time that he is trying to browbeat a foreign leader to do something illicit, to provide dirt on his opponent during a presidential campaign, then that (impeachment) may be the only remedy that is co-equal to the evil that conduct represents."


Members of congress are also looking into whether Mr Trump's lawyer Rudy Giuliani travelled to Ukraine to pressure the government to aid Mr Trump's re-election effort by investigating the activities of Mr Biden's son.

During an interview on CNN, Mr Giuliani was asked whether he had asked Ukraine to look into Mr Biden. He initially said: "No, actually I didn't," but seconds later he said: "Of course I did."

Image: There have been calls for Joe Biden to be investigated

Mr Biden is a frontrunner for the 2020 Democratic presidential nomination,

Other Democrats, including fellow White House hopeful Elizabeth Warren, and New York congresswoman Alexandria Ocasio Cortez, have called for Democratic leadership to pursue impeachment immediately.

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Mr Trump has dismissed the matter, insisting he did nothing wrong while pointing the finger at Mr Biden.

The US president said: "What he [Biden] said is that he wouldn't give, I think it was billions of dollars, to Ukraine unless they fired the prosecutor who is looking at his son and his son's company, the company that his son worked with, and that's a very dishonest thing.

"And I'm not looking to hurt Biden. But he said a very bad thing. He said a very foolish thing. Now me on the other hand, my conversation with the president, the new president of Ukraine was perfect."

Image: Volodymyr Zelenskiy is Ukraine's president

Meanwhile, US Secretary of State Mike Pompeo called on Mr Biden to be investigated if evidence shows he improperly intervened to protect his son from an investigation in Ukraine.

"I do think if vice president Biden behaved inappropriately, if he was protecting his son and intervened with the Ukrainian leadership in a way that was corrupt, we need to get to the bottom of that," he said on ABC's This Week.

On Saturday, Mr Biden called for an investigation into reports that Mr Trump pressed his Ukrainian counterpart to investigate him and his son.

Mr Biden said: "This appears to be an overwhelming abuse of power. To get on the phone with a foreign leader who is looking for help from the United States and ask about me and imply things this is outrageous."

Impeachment proceedings in Congress can lead to a president being removed from office, but Democrats would need support from Mr Trump's fellow Republicans who control the Senate.