U.S. President Donald Trump on Monday said he is "strongly" considering answering a challenge from opposition Democrats to testify in his own impeachment investigation. 

"Even though I did nothing wrong, and don't like giving credibility to this No Due Process Hoax, I like the idea & will, in order to get Congress focused again, strongly consider it!" Trump tweeted.  

After the speaker of the lower house, Nancy Pelosi, upped the ante by suggesting that Trump come forward to tell the "truth," the Republican president said he was keen. 

The impeachment inquiry of Trump shifts into higher gear this week when a parade of officials will face questioning by Democratic lawmakers seeking details that could link Trump to a pressure campaign against Ukraine potentially for his political benefit. 

Eight more witnesses are due to testify in the second week of the televised hearings. They include Gordon Sondland, the U.S. ambassador to the European Union, whose direct interactions with Trump are likely to be a main focus in the investigation of whether the president made security aid to Ukraine contingent on it agreeing to dig up dirt on a political rival, former Vice President Joe Biden. 

The latest round of hearings will stretch from Tuesday to Thursday before the House of Representatives Intelligence Committee. Seeking to build on last week's testimony by three key witnesses, Democrats leading the inquiry - the first public impeachment drama in two decades - will continue trying to make the case that Trump abused the power of his office. 

Denying any wrongdoing, Trump, who railed on Twitter and elsewhere against the proceedings and attacked witnesses by name last week and over the weekend, has shown no sign of a let-up in his confrontational approach. Some Democrats have accused him of witness intimidation but most Republican lawmakers have joined him in declaring the inquiry unfair.

Several witnesses testified last week that they were alarmed over the pressure tactics used against Ukraine, as well as the role of Trump's personal lawyer, Rudy Giuliani. 

At the heart of the inquiry is a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelensky to open a corruption investigation into Biden and his son, Hunter Biden, and into a discredited theory that Ukraine, not Russia, meddled in the 2016 U.S. election. 

Among this week's witnesses are several who listened in on the call. 

Lieutenant Colonel Alexander Vindman, a Ukraine expert at the National Security Council (NSC), testified behind closed doors last month that he was so concerned about efforts to push Ukraine to investigate a Trump rival that he reported it to the NSC's lawyer. 

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Fiona Hill, Trump's former top Russia adviser, who also testified in private last month and is now set to appear on Thursday, previously recounted how U.S. policy on Ukraine got caught up in clashes between official and unofficial channels. 

Due to testify on Tuesday are Kurt Volker, former U.S. special envoy to Ukraine; Jennifer Williams, an adviser to Vice President Mike Pence; and Tim Morrison, an NSC aide. Wednesday will see Laura Cooper, the deputy assistant secretary of defense for Russia, Ukraine and Eurasia, and David Hale, the undersecretary of state for political affairs, at the witness table. 

The hearings could pave the way for the Democratic-led House to approve articles of impeachment - formal charges - against Trump. That would lead to a trial in the Senate on whether to convict Trump and remove him from office. Republicans control the Senate and have shown little support for Trump's removal.

(Cover: U.S. President Donald Trump delivers remarks at a campaign rally in Bossier city, U.S., November 14, 2019. /VCG Photo)

(With input from AFP, Reuters)