SWIFT, the global financial messaging system, on Tuesday disclosed new hacking attacks on its members' banks as it pressured them to comply with security procedures instituted after February's high-profile $81 million heist at Bangladesh Bank.

In a private letter to clients, SWIFT said new cybertheft attempts — some of them successful — have surfaced since June, when it last updated customers on a string of attacks discovered after the attack on the Bangladesh central bank.

"Customers' environments have been compromised, and subsequent attempts made to send fraudulent payment instructions," according to a copy of the letter. "The threat is persistent, adaptive and sophisticated — and it is here to stay."