Pompeo Claims Russia is 'Pretty Clearly' Behind Major Hack Attack on US

"There was a significant effort to use a piece of third-party software to essentially embed code inside of US government systems", Pompeo told The Mark Levin Show.

"This was a very significant effort, and I think it's the case that now we can say pretty clearly that it was the Russians that engaged in this activity".

Moscow rejected the accusations, noting that no evidence had been provided to back up the claims of "Russian hackers" attacking the computer systems.

The hackers reportedly managed to breach the security of the US government agencies by compromising the Texas-based company SolarWinds' software.

The Washington Post said that a hacking group called APT29, also known as "the Dukes" or "Cozy Bear" was behind the incidents.

Russian President Vladimir Putin addressed the reports during his annual press conference on 17 December, noting that the anonymous sources, cited by US media, are American officials and intelligence agencies, who previously pushed groundless accusations against Russia following the 2016 presidential elections.