South Korean authorities are investigating a cyber attack which brought down the servers of two major banks on 20 March.
The Korea Internet Security Agency said computer networks at Shinhan Bank and Nonghyup Bank had been severely disrupted as a result of the attack.
Shinhan Bank reported a two-hour system shutdown that affected online banking and ATMs.
The bank said systems had been restored and that no customer information had been compromised.
Authorities investigating the cause of the system failures and the national security office declined to comment on where the attack may have originated, although speculation centres on North Korea.