Japan Net Bank Ltd. is offering customers a "one-time debit" service to combat growing online card fraud.

Japan Net Bank account holders can request a one-time-use Visa Inc. card number that is valid for 10 days, and may use it at most online stores that accept Visa. The Tokyo online banking company will automatically transfer payments from users accounts to cover transactions of up to $1,120.

Not everyone is convinced the virtual card will provide adequate security. Though the service is innovative, there still is no guarantee payments will be safe, said Kei Shimada, the chief executive of the Tokyo research firm Infinita Inc.

"The payment is simultaneous with the transaction, meaning it won't be an automatic withdrawal at month-end as with most transactions on credit cards and bank accounts," he said. "So, even if there is an event where the transaction is a forged one, it may delay the timing in which users realize the fraudulent transaction."

Japan Net promises to compensate card users up to $56,500 for fraudulent transactions reported within 30 days, but this is a less than standard credit cards, which typically allow up to 60 days to report fraud, Shimada said.