Cyber security firm Symantec said on Monday it was "highly likely" a hacking group affiliated with North Korea was responsible for the WannaCry cyber attack earlier this month.

Previous versions of the worm used in targeted attacks earlier this year were "almost identical" to the one that spread to more than 150 countries and used hacking tools and infrastructure seen in previous cyber operations linked to the Lazarus hacking group, Symantec said.

Lazarus has been previously linked to North Korea by the U.S. government and cyber security researchers.

The attacks infected more than 300,000 computers worldwide and disrupted operations at hospitals, banks and schools across the globe.