The operator of 7-Eleven convenience stores said Thursday that some 900 customers using its mobile payment service had lost a total of ¥55 million ($510,000) due to unauthorized access to their accounts.

The problem surfaced soon after the operator, Seven & I Holdings Co., launched the smartphone payment service, 7pay, at over 20,000 stores across the country on Monday.

Tsuyoshi Kobayashi, president of Seven Pay Co., told a news conference in Tokyo that the company will compensate users for the losses caused by fraudulent access. He added that new user registrations were already suspended, as well as functions that allow users to add funds that can be spent using the smartphone application.

According to Seven & I Holdings, some customers reported losses Tuesday. The parent company confirmed that someone had accessed their accounts, used the details of their registered credit or debit cards and purchased items at its convenience stores.

Rival convenience store firm FamilyMart Co. has also launched a smartphone payment service, FamiPay, in a bid to attract more customers amid intensifying competition.

The retail industry has been beset by similar breaches in the past. The online stores of Fast Retailing Co., the operator of Uniqlo, were hacked between April and May, leading to the possible disclosure of personal information from about 460,000 customers.

Around 700 accounts held by customers of retail giant Aeon Co.’s banking and credit card service were subject to unauthorized access from late May to early June, with losses exceeding ¥22 million.