Washington (AFP) - -President Donald Trump's convicted former national security advisor Michael Flynn and former top campaign aide Rick Gates have been subpoenaed to testify to Congress, the House Intelligence Committee announced Thursday.

Flynn and Gates, both key witnesses in the Russian meddling investigation of Special Counsel Robert Mueller, were ordered to appear before the committee for their probe into security threats arising from links between the Trump campaign and the Russian government.

Gates, who pleaded guilty to lying and to financial crimes, had earlier refused to voluntarily meet with the committee and supply documents.

Flynn, the former Pentagon intelligence chief who is awaiting sentencing on a charge of lying to investigators about his contacts with Russia, has also avoided testifying.

Both are being asked to take part in a series of televised hearings convened by Democratic committee chairman Adam Schiff that aim to present details of Mueller's investigation into alleged collusion with Russia to the public.

The principal aim of the initial hearings this week has been to undermine Trump's claim that there was nothing wrong with the contacts between members of his campaign team and Russians, including their effort to seek dirt on Trump's election rival Hillary Clinton.

Mueller's report documented numerous contacts, some of which appeared to be active collusion, but said he could not show evidence of a criminal conspiracy.

"As part of our oversight work, the House Intelligence Committee is continuing to examine the deep counterintelligence concerns raised in Special Counsel Mueller’s report, and that requires speaking directly with the fact witnesses," Schiff said in a statement.

"The American people, and the Congress, deserve to hear directly from these two critical witnesses."