US President Donald Trump has said he is "strongly" considering answering a challenge from opposition Democrats to testify in his own impeachment investigation.

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

He tweeted that Ms Pelosi suggested "that I testify about the phony Impeachment Witch Hunt. She also said I could do it in writing.

"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!"

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

Eight more witnesses are due to testify in televised hearings.

They include Gordon Sondland, the US ambassador to the European Union, whose direct interactions with Mr 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.

Denying any wrongdoing, Mr 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 Republicans 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 Mr Trump’s personal lawyer, Rudy Giuliani.

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

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

Due to testify tomorrow are Kurt Volker, former US 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 defence 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 Mr Trump.

That would lead to a trial in the Senate on whether to convict Mr Trump and remove him from office. Republicans control the Senate and have shown little support for his removal.