Cybersecurity firm Symantec has found evidence that North Korea is behind the recent string of attacks on several Asian banks.

Symantec said the malware used to steal $81 million from Bangladesh's central bank is linked to attacks on a bank in the Philippines and in Vietnam.

This may be the first time one country has used malicious code to steal money from another country.

Security researchers say the malware is similar to that used in the past by a group known as "Lazarus." The group has been linked to a string of hackings largely focused on U.S. and South Korean targets dating back to 2009. That includes the crippling 2014 hack of Sony Pictures, which the FBI has blamed on the North Korean government. North Korea denied the allegation.