The witnesses are asked by Washington state Democrat Denny Heck why it would matter that Russia allegedly meddled in the US election campaign.

FBI Director Comey says: "When there’s an effort by a foreign nation state to mess with that (US democracy), to destroy that, to corrupt that, it’s very, very serious, threatens what is America.

"If any Americans are part of that effort that’s a very serious matter and so you would expect the FBI to understand if that’s so, and if so who did what."

NSA chief Mike Rogers says: "This behaviour is not likely to stop, this is not likely to be the last time we have a discussion about this kind of activity."