The Irish Payment Services Organisation is reassuring hundreds of bank customers hit by an online scam that they will not be liable for fraudulent transactions on their credit cards.

It is believed criminals accessed the credit card details several months ago, but have begun to use them only in the past two days.

The monitoring system operated by Irish banks spotted the unusual transactions, after the card details were used to buy low-value items, for €1 or €2, on a US website. It is thought the criminals were testing the cards to check if they were still valid.

The banks have been contacting customers affected, blocking their cards and re-issuing them with new ones. An investigation is continuing into how the credit card details were stolen.

Una Dillon of the Irish Payment Services Organisation (IPSO) says anyone who has not already been contacted by their bank at this stage is unlikely to have been affected by this scam. She advised people who are worried that their card may have been compromised to contact the card issuer, and check their bank account for unusual activity.