WASHINGTON (BLOOMBERG) - Elizabeth Warren pressed Congress to "step up" and begin impeachment proceedings against President Donald Trump, after reports that the president asked Ukraine to investigate Joe Biden's son.

"What the president has now demonstrated is that he thinks it's pretty clear, he doesn't have to follow the law and in fact can continue to commit high crimes and misdemeanours," Warren told reporters in Cedar Rapids, Iowa, on Friday (Sept 20).

"And that is invite and profit from foreign interference in our election. It's time for Congress to step up and begin serious impeachment proceedings against this."

Trump held a phone call on July 25 with Ukraine's new president, where he pressed Volodymyr Zelenskiy to investigate Biden's son, Hunter, according to a person familiar with the call.

Biden condemned the reports and called on Trump to release the transcript of the Zelenskiy phone call.

"It means that he used the power and resources of the United States to pressure a sovereign nation - a partner that is still under direct assault from Russia - pushing Ukraine to subvert the rule of law in the express hope of extracting a political favour," Biden said on Friday in a statement.

A majority of House Democrats wants to begin impeachment proceedings, though Speaker Nancy Pelosi has so far refused to move forward, fearing it would be harmful to Democratic candidates seeking re-election in Republican-leaning areas in 2020. There also is little prospect that any effort would be successful in the Republican-controlled Senate.

The details of a whistle-blower complaint from the intelligence community about the White House remain murky, but some of the details may suggest a reprise of the scandal over Hillary Clinton's improper use of a private email server during the 2016 election. Trump repeatedly suggested she had deleted a huge trove of e-mails to cover up wrongdoing. This time, the target could be Democratic front runner Joe Biden via the dealings of his son Hunter in Ukraine.

According to reports in the New York Times and the Washington Post, the whistle-blower complaint partly involves a phone call in which Trump made some kind of commitment to a foreign leader that involves Ukraine.

In tweets, Trump maintained that the call was a "perfectly fine and respectful conversation" and referred to "'highly partisan' whistle-blowers." And speaking at the White House on Friday, Trump said "it doesn't matter" if he asked Ukraine to look into Biden and argued it should be done regardless. "Someone ought to look into Joe Biden," he said.

Related Story Biden clashes with Warren, Sanders on healthcare in Democratic 2020 debate

Related Story Presidential hopeful Elizabeth Warren says US should use trade deals to raise global standards

To add to the intrigue, Trump lawyer and confidant Rudy Giuliani told CNN on Thursday that he had asked the Ukrainian government to open an investigation into Hunter Biden's business dealings in that country. "Of course I did," he said on Thursday.

For months, Trump has argued that Biden improperly pressured Ukraine's top prosecutor to drop an investigation into a company that Hunter Biden was involved in. There's no evidence to back up that assertion, and the current prosecutor has said that he does not "see any wrongdoing" by either Biden.