Apple iPhone 6S Plus
A man plays with the iPhone 6s Plus at Apple Store, Sept. 25, 2015 in Sydney, Australia. Cole Bennetts/Getty Images

Apple may be forced to replace far more iPhone 6S batteries than it originally planned. The company announced last month that a small number of phones manufactured between September and October 2015 had a battery issue that was causing devices to shut down unexpectedly. As a redress, it offered customers who had purchased phones potentially affected by the problem the chance to take in their devices and get a replacement battery free of charge.

However, Apple conceded on its Chinese Support page Tuesday that the issue may not be limited to phones produced within that limited time window.

"A small number of customers with phones outside of the affected range have also reported an unexpected shutdown,” read the message. “Some of these shutdowns can occur under normal conditions in order for the iPhone to protect its electronics.”

Apple went on to say that it would be adding a diagnostic tool to a software update due to be released next week in order to try and gather more information on the issue. A fix could then be added to further updates.

The technology giant again stressed that the issue was not a safety one. Earlier this year, its major rival in the smartphone market, Samsung, was compelled to recall its new Galaxy Note 7 phone after hundreds of devices worldwide saw their batteries catch fire or even explode. When replacement units began seeing the same problem, Samsung told users to shut down their phones and in October issued a recall of all Galaxy Note7s, including replacements.

Samsung estimated the cost of the whole episode to its operating profit would be $3 billion.

Apple’s problems have not yet been anywhere near as serious. Yet, its iPhone issues have extended beyond the 6S just in the last few weeks. Last month, the company announced a “multitouch repair program” for the iPhone 6 Plus to cure what has been termed the “touch disease. Users wanting the issue fixed still have to pay $149.

In the case of the battery problem with the iPhone 6S, those with affected units can go to an Apple Store or authorized service providers, including Best Buy, to get a replacement.