A top Democrat in the US House of Representatives has accused former National Security advisor Michael Flynn of lying to investigators when he was being checked for high level security clearance.

In a letter to the chairman of the House Committee on Oversight and Government Reform, Representative Elijah Cummings said that the committee "has in our possession documents that appear to indicate that General Flynn lied to the investigators who interviewed him in 2016 as part of his security clearance renewal".

The accusation adds another facet to an already convoluted story, coming just hours after it was reported that Flynn would not honour a subpoena from Congress and would instead invoke the fifth amendment saying that there was an "escalating public frenzy against him" and it was not in his interests to cooperate.

In the letter addressed to Oversight Committee chairman Jason Chaffetz, Cummings says the Flynn told investigators that trips he took to Moscow were funded by US companies when they were actually funded by "the Russian media propaganda arm, RT."

Commenting on Flynn's decision not to comply with the subpoena, Senate Intellgence Committee chairman, Senator Richard Burr, and Vice-chairman, Mark Warner, issued a joint statement saying that while they recognise Flynn's right to invoke the fifth amendment, "we are disappointed he has chosen to disregard the Committee's subpoena request for documents relevant and necessary to our investigation. We will vigorously pursue General Flynn's testimony and his production of any and all pertinent materials pursuant to the Committee's authorities."

Michael Flynn was unceremoniously fired from the administration weeks after taking the position when it was reported that he had had phone calls discussing sanctions with the Russian ambassador before he had taken public office. The administration now faces questions as to how much was known about Flynn's background before he was offered the position.

Ex-National security adviser General Michael Flynn arrives to deliver a statement during the daily briefing at the White House in Washington DC, 1 February 2017 Reuters/Carlos Barria