Ulster Bank has said it is seeing increased levels of fraudulent activity on some of its customers' debit cards in recent days.
If follows reports from customers that their cards had been successfully targeted by fraudsters.
The bank said the scammers are putting through false, low value transactions on the cards.
Some customers reported transactions for Google's Play store which they had not been responsible for had registered on their cards.
Others said their cards had been used to pay for something in a restaurant in Canada.
"We are working to identify and block these transactions; reissue cards to impacted customers if required; and provide refunds," a spokesperson for the bank said.
"We apologise for the inconvenience, especially over the Christmas period."
It is understood that no customer will be left out of pocket by the bank as a result of such fraudulent activity.
Customers can contact Ulster Bank on 0818 210 260 if they have any concerns about potentially fraudulent activity on their account.
"Due to increased call volumes, wait times may be a longer than normal, we are working hard to resolve this and no customer will be left out of pocket," it said.
The spokesperson added that it is not yet clear how the fraudsters were able to get the card data.
In one case a card had been reissued this week following a similar fraud at the weekend and used just once before another fraudulent transaction was put through by scammers.
Banks are in a constant battle to prevent online card transaction fraud from taking place and regularly warn customers to be careful with their card details, particularly when using them online.
The problem is most acute at times of the year when transaction volumes increase, such as Christmas.
Last week, Bank of Ireland said it was temporarily reducing its maximum combined payment limit for new payees to €1,000 per working day.
It said it was taking these measures to protect customers, as fraudsters have been targeting customers with fake texts and emails.
"They have tricked people into giving them their online banking or card details and have stolen from our customers," it said in an email to customers.
"We apologise for any inconvenience this may cause for you, but temporarily lowering limits helps to protect your money."
Forty-eight hours after a new payee is added, the standard combined daily limit increases to €20,000, Bank of Ireland said.
