Russia was "pretty clearly" behind a devastating cyberattack on several US government agencies that also hit targets worldwide, Secretary of State Mike Pompeo said. 

Microsoft said late Thursday that it had notified more than 40 customers hit by the malware, which security experts say could allow attackers unfettered network access to key government systems and electric power grids and other utilities.

"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 on Friday.

"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."

Roughly 80 percent of the affected customers are located in the United States, Microsoft president Brad Smith said in a blog post, with victims also found in Belgium, Britain, Canada, Israel, Mexico, Spain and the United Arab Emirates.

James Lewis, vice president at the Center for Strategic and International Studies, said the attack may end up being the worst to hit the United States, eclipsing the 2014 hack of US government personnel records in a suspected Chinese infiltration.

Russia has denied involvement.