WASHINGTON (AFP) - President Donald Trump on Monday 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 Trump come forward to tell the "truth," the Republican president said he was keen.

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

"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!" It was unclear what kind of testimony the president had in mind.

His defence team would likely be highly resistant to the idea of him appearing before the House intelligence committee delving into the allegation that Trump pressured Ukraine to dig up dirt on a political rival to help his 2020 re-election chances.

During the lengthy probe led by special prosecutor Robert Mueller into accusations that Trump worked with Russians to boost his 2016 election chances, the president answered questions in written form.

However, he only agreed to the step after his lawyers negotiated strict limits on what kind of questions could be put. In dozens of instances, Trump then said he could not "recall" the facts.

The Mueller report ultimately found that Russian agents had sought to influence the US election but that there was no evidence of collusion with the Trump campaign.