A Democrat representative has strongly criticised Mike Flynn's reported communication with Russia ahead of Donald Trump's election win describing "conspiring" with Russia as "the definition of treason".
Flynn resigned on 13 February, less than a month into his role as national security adviser, after it emerged he had spoken with Russia's ambassador to the US about the possible easing of sanctions prior to Trump taking office.
Congressman Seth Moulton, who is a member of the House Armed Services Committee, said if members of the US administration were "conspiring" with Russia, it was putting the "enemy's" needs ahead of those of the US.
Speaking to CNN host Wolf Blitzer, the Democrat said, in comments carried by The Hill: "I mean, let's not lose perspective on exactly who we're talking about here. Russia is the number one enemy of the United States of America.
"If members of the administration are essentially conspiring with Russia – either through the campaign earlier or now in the administration itself, I mean, look, Wolf, that's the very definition of treason.
"This is a very, very serious affair. The definition of treason is putting our enemy's interests ahead of our own. That's what the definition is."
The episode looks particularly bad for an administration fending off accusations of clandestine links to Russia, including conclusions reached by the US intelligence community that the Kremlin ordered hackers to interfere with last year's election to help sway it in Trump's favour.
"It seems like there's a lot of evidence that there are members of this administration that are more concerned with Russia's goals than our own," Moulton added in his CNN interview.
"They continually prioritise the desires of the Kremlin, like, for example, rescinding these sanctions that have been so important."
Flynn's suitability for the position was brought into question soon after he was named. During the election campaign, he was found to have tweeted out fake news conspiracy theories about Hillary Clinton - something for which his son was fired from the Trump transition team.