A problem with iPhone 6s products randomly shutting down comes from a battery flaw found in a small number of models, according to Apple.
After a Chinese consumer watchdog group reported the issue, Apple is offering a fix free of charge to any eligible iPhone 6s user through its customer support sites.
On Friday, Apple explained on its Chinese site that the problem was found in iPhone 6s devices containing a faulty battery component. This component was "exposed to controlled ambient air longer than it should have" before it was assembled into the battery packs, Apple said.
"As a result, these batteries degrade faster than a normal battery and cause unexpected shutdowns to occur," the company said.
However, the faulty components were only found only in a "small number" of iPhone 6s devices made in September and October 2015.
"It's important to note, this is not a safety issue," Apple said. Its iPhones are built to automatically shut down under certain conditions, such as extremely cold temperature, to protect the device's electronics.
Apple has posted customer support websites where consumers can submit their iPhone 6s serial numbers to see if they are eligible for a battery replacement.
Last month, the China Consumers Association asked the company to investigate the shutdown issues, after receiving a "considerable number" of complaints from iPhone users in the country.
The shutdowns would occur when the device’s battery charge dropped between 60 to 50 percent in both cold and room-temperature environments, according to the association. The phones would also fail to turn on unless connected to a power supply.
Although Apple claims to have addressed the problem, the China Consumers Association said earlier this week that consumers continue to complain the shutdown issues are also found in older iPhone 6 and 6 plus models.
In addition, consumers with iPhone 6s models built outside the September and October 2015 date range have also reported the problem.
The China Consumers Association has asked Apple to investigate the other claims. But on Friday, Apple said it had found no other problems.
"We looked for any other factors that could cause an iPhone to shut down unexpectedly," the company said. "After intensive investigations, no new factors have been identified. We will continue to monitor and analyze customer reports."