The personal data of 175,000 more customers were found to have been leaked from credit card firms that were responsible for a massive data theft earlier this year, financial sources said Tuesday.

The classified data of up to 140,000 shop owners were leaked from KB Kookmin Card, along with information on 35,000 customers of NH Nonghyup Card, according to the sources.

The additional breach comes just months after some 103 million cases of data leak were discovered at KB Card, NH Card and Lotte Card.

The investigators said after a separate probe in March that they found 80 million more cases spanning over different periods.

The Financial Supervisory Service (FSS) continued the probe by comparing the two sets of results and found additional information thefts that included names, citizen identification numbers, contacts and employment details, the sources said.

The FSS said it will beef up monitoring on customer information security and protection at financial firms to prevent personal data from spreading to financial scammers.

By Bae Hyun-jung and news reports (tellme@heraldcorp.com)