A North Korean hacking group was likely behind a recent cyber campaign targeting organisations in 31 countries, cyber security firm Symantec Corp says.
The company said in a blog that researchers have uncovered four pieces of digital evidence suggesting a group, known as Lazarus, was behind the attacks.
The cyber campaign sought to infect victims with "loader" software used to stage attacks by installing other malicious programs.
"We are reasonably certain" Lazarus was responsible, Symantec researcher Eric Chien said in an interview.
The North Korean government has denied allegations it was involved in the hacks, which were made by officials in Washington and Seoul, as well as security firms.
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US Federal Bureau of Investigation representatives could not immediately be reached for comment.
Symantec did not identify targeted organisations and said it did not know if any money had been stolen.
Nonetheless, Symantec said the claim was significant because the group used a more sophisticated targeting approach than in previous campaigns.
Lazarus has already been blamed for a string of hacks dating to at least 2009, including last year's $US81 million heist from Bangladesh's central bank, the 2014 hack of Sony Pictures Entertainment that crippled its network for weeks and a long-running campaign against organisations in South Korea.
