Cyberattacks targeting the global bank transfer system have succeeded in stealing funds since February's heist of $81 million from the Bangladesh central bank as hackers have become more sophisticated in their tactics, according to a SWIFT official and a previously undisclosed letter the organization sent to banks worldwide.

The messaging network in a Nov 2 letter seen by Reuters warned banks of the escalating threat to their systems, according to the SWIFT letter.

The attacks and new hacking tactics underscore the continuing vulnerability of the SWIFT messaging network, which handles trillions of dollars in fund transfers daily.