Kobe Steel's fake data scandal expanded to its core business after the Japanese company admitted "inappropriate actions" related to steel wire produced overseas, triggering a fresh collapse in its shares and heightened speculation that the steelmaker may get broken up.

Customers have been informed about the issue, which has been resolved, Tokyo-based spokeswoman Eimi Hamano said by phone, declining to provide details. Kobe Steel falsified quality certification data for steel wire used in auto engines and to strengthen tires, the Nikkei newspaper reported Friday.

Kobe's admission of misconduct in its steel business, which accounts for about a third of revenue, ratchets up the pressure on Japan's third-biggest steelmaker. The company's disclosures had up until now dealt with aluminum, copper and iron ore products used in everything from cars to computer hard drives to Japan's iconic bullet trains, although there haven't been any reports of products being recalled or safety concerns raised.