Cyber security researchers have found technical clues they said could link North Korea with the global WannaCry "ransomware" cyber attack that has infected more than 300,000 machines in 150 countries since Friday.

Symantec and Kaspersky Lab said on Monday some code in an earlier version of the WannaCry software had also appeared in programs used by the Lazarus Group, which researchers from many companies have identified as a North Korea-run hacking operation.

"This is the best clue we have seen to date as to the origins of WannaCry," Kaspersky Lab researcher Kurt Baumgartner told Reuters.

“At this time, all we have is a temporal link,” Eric Chien, an investigator at Symantec, told the New York Times. “We want to see more coding similarities to give us more confidence.’’