Unknown hackers stole 339.5 million roubles ($6 million) in an attack on the SWIFT international payments messaging system in Russia last year, the Russian central bank said on Friday.

The disclosure, buried at the bottom of a central bank report on digital thefts at Russian banks, is the latest in a string of attempted and successful cyber heists using fraudulent wire-transfer requests.

The central bank said it had been sent information about "one successful attack on the work place of a SWIFT system operator.

"The volume of unsanctioned operations as a result of this attack amounted to 339.5 million roubles," the bank said.

The central bank declined to provide further details.

A spokeswoman for SWIFT, whose messaging system is used to transfer trillions of dollars each day, said the company does not comment on specific entities.

"When a case of potential fraud is reported to us, we offer our assistance to the affected user to help secure its environment,” said the spokeswoman, Natasha de Teran.

A central bank spokesman quoted Artem Sychev, deputy head of the regulator's security department, as saying the hackers had withdrawn the money and this was "a common scheme, when they take control of a computer."

Brussels-based SWIFT said late last year digital heists were becoming increasingly prominent as hackers use more sophisticated tools and techniques to launch new attacks .

In December, hackers tried to steal 55 million roubles from Russian state bank Globex using the SWIFT system, and digital thieves made off with $81 million from Bangladesh Bank in February 2016.

SWIFT has declined to disclose the number of attacks or identify any victims, but details on some cases have become public, including attacks on Taiwan's Far Eastern International Bank and Nepal's NIC Asia Bank. ($1 = 56.7140 roubles)