U.S. President Donald Trump has ramped up his bellicose rhetoric as the Democratic-led House of Representatives moved ahead with its impeachment inquiry.

Trump suggested on September 30 that Adam Schiff, the California Democrat who is leading the inquiry, should be arrested for 'treason.'

The congressional probe was prompted by a whistle-blower's account that Trump pressured Ukrainian President Volodymyr Zelenskiy to investigate former Vice President Joe Biden, currently a Democratic front-runner in next year's presidential election.

The whistle-blower's complaint cited a July 25 phone call between Trump and Zelenskiy during which the U.S. president allegedly pushed his Ukrainian counterpart to investigate Biden and his son Hunter, who sat on the board of a Ukrainian gas company when his father worked in then-President Barack Obama's administration.

No evidence of wrongdoing has surfaced regarding either of the Bidens.

Secretary of State Mike Pompeo was also on the July 25 call with Zelenskiy, The Wall Street Journal, Reuters, and AP reported.

The Trump-Zelenskiy phone call came shortly after the United States withheld almost $400 million in military funding to Ukraine, causing concern that the president was using the aid approved by Congress as a bargaining chip for his personal advantage.

In a series of Twitter posts on September 30, Trump appeared to refer to comments made by Schiff, the House Intelligence Committee chairman, during a committee hearing last week.

Schiff 'illegally made up a FAKE & terrible statement, pretended it to be mine as the most important part of my call to the Ukrainian President, and read it aloud to Congress and the American people. It bore NO relationship to what I said on the call. Arrest for Treason?' Trump tweeted.

The post followed a series of other posts from Trump in which the president said he wanted to face the whistle-blower, who he called 'my accuser,' and 'the person who illegally gave this information' to him or her.

The whistle-blower hasn't been publicly identified.

Witnesses are due to testify in the House this week in hearings related to the impeachment investigation, including former U.S. Ambassador to Ukraine Marie Yovanovitch and Kurt Volker, who resigned last week as Trump's special representative for Ukraine.

Trump's personal lawyer, Rudy Giuliani, was subpoenaed on September 30 and asked to provide documents related to the president's alleged effort to pressure Zelenskiy to investigate Biden.

Additional subpoenas were issued for businessman Lev Parnas and real estate investor Igor Fruman, who, according to several media accounts, facilitated introductions between Giuliani and top Ukrainian politicians.

Another summons was sent to Semyon 'Sam' Kislin, a Ukrainian immigrant who had business ties to Trump and served under Giuliani when he was mayor of New York.

Meanwhile, several U.S. media outlets reported that U.S. Attorney General William Barr asked foreign intelligence officials to help with a Justice Department inquiry into the origins of Special Counsel Robert Mueller's Russia probe that found evidence of Russian interference in the 2016 presidential election, according to CBS News and The Washington Post.

British intelligence officials were asked for help and last week Barr flew to Italy to meet with senior Italian government officials. Simultaneously, Trump asked the Australian prime minister and other foreign leaders to help Barr with his investigation into the Russia probe.

Justice spokeswoman Kerri Kupec told AP that Trump made the calls on Barr's behalf.

With reporting by Reuters, AFP, AP, The Washington Post, New York Times, Wall Street Journal, and CBS News

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