Citigroup's (C) certain customers, who hold its Strata Elite card, were locked out of their accounts without explanation for weeks, The Wall Street Journal reported Thursday, quoting affected customers.

These customers were unable to make purchases or redeem rewards that cost an annual fee of $595, the report said.

To unlock the cards and rewards, the bank requires customers to submit an income-verification form from the Internal Revenue Service, the Journal said.

Citi expressed regret for "the inconvenience some of our cardmembers have experienced" and stated that protecting accounts was its top priority, the report said.

Citigroup did not immediately reply to an MT Newswires request for comment.

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