Lawmakers and senior US intelligence officials have drawn a line in the sand for Donald Trump.

They've presented a united front on their conclusion that Russia is a major threat to the United States and was behind election hacking.

Three top intelligence officials have made a joint statement to the Senate Armed Services Committee.

They say only Russia's senior-most officials could have authorised the data thefts and disclosures.

Committee Chair John McCain has emphasised the Russian role and delivered an implicit rebuke to Mr Trump ... who has urged people to "move on" from the issue.

"Every American should be alarmed by Russia's attacks on our nation. There is no national security interest more vital to the United States of America than the ability to hold free and fair elections without foreign interference."

Director of National Intelligence James Clapper was asked directly about Mr Trump's criticism.

"I think there is an important distinction here between healthy scepticism, which policy makers should always have for intelligence, but I think there's a difference between scepticism and disparagement

Mr Clapper said he's received many expressions of concern from foreign counterparts about it.