First Niagara Financial Group is replacing debit cards for a “small number” of its customers following a security breach at an unidentified merchant.

“This is not a breach of First Niagara systems; our systems are secure,” the bank said in a statement. “Out of an abundance of caution, we are reaching out to affected customers to notify them and to replace affected cards. No customer will incur a loss as a result of this potentially fraudulent activity.”

First Niagara said it could not identify the merchant where the breach occurred, due to a confidentiality agreement.

The breach occurred in June, said Jodi Johnston, a First Niagara spokeswoman. The Buffalo-based bank said less than 1 percent of its customers were issued new cards.

“Customers who were affected already know,”she said.

Asked about the breach’s impact on KeyBank, a representative said: “KeyBank has numerous prevention and detection controls that mitigate fraud risk. We’re not seeing anything out of the ordinary in the Buffalo area.”