A sharp rise in the number of illegal withdrawals from online bank accounts has been reported in recent months, according to the National Police Agency.

In November, 573 cases of unauthorized withdrawals from internet banking accounts and money transfers, amounting to ¥776 million, were reported, compared with 105 cases and ¥67 million in August, data released by the agency last week showed.

In July, the number of cases totaled 48.

The Japanese Bankers Association, which shares information with the agency, has been warning users not to open "suspicious" emails or other digital text messages and delete them as soon as possible.

A typical trick used by defrauders is to send text messages at random and make them look as if they have been sent by the banks. In the messages, users are asked to re-enter their passwords for savings accounts and other personal information for security enhancement.

Such messages are normally designed to route them to fake websites of online bank operators.

Those phishing sites even stole two-factor authentication passwords, enabling fraudsters to transfer stolen money to another account within a short time, according to the agency.