Details of more than 3.2 million cash cards of customers of top Indian banks have reportedly been stolen in what could be one of the biggest financial data breaches in the country. While it is unclear who was behind the alleged operation, reports suggest that unauthorised transactions can be traced to various locations in China.

The alleged breach was first reported by Indian daily Economic Times, following which banks have started blocking cards of certain customers. Major financial institutions such as State Bank of India, ICICI Bank, HDFC, YES Bank and Axis Bank are believed to be the worst affected by the breach.

According to reports, some customers' cards have been blocked, while some customers have been asked to change the pin number of their ATM cards.

"We have received complaints from banks about debit cards being used in China which aroused suspicion. Though most of the suspected fraudulent transactions happened in the Visa and MasterCard network, we thought a whole a forensic audit of the entire network will help us find out where the compromise happened," said AP Hota, managing director of National Payments of Corporation of India – the umbrella organisation of retail payments in the country.

Government-owned State Bank of India, which is the nation's biggest lender by assets, acknowledged cards of its customers were breached and said that it was addressing concerns. Yes Bank said there was no evidence of data breach, but it was stepping up the security measure proactively.

While 2.6 million cards were on either the Visa or MasterCard platform, more than 600,000 bank customers were on the domestic card scheme RuPay. "Our internal monitoring mechanism identified such a threat recently and all steps have been undertaken to neutralize the same," read a statement from Axis Bank.

In September, several banks – which also feature in the latest list – sent out messages to their customers urging them to change their ATM pins as a precautionary measure. In India, it is estimated that there are roughly 700 million debit cards issued by various banks.