In this March 24, 2016, file photo, from left, the iPhone 6S Plus, 6S and SE lie next to each other in a comparison in an Apple store in New York. [Photo/IC]

US tech giant Apple Inc Friday announced the reason behind the abrupt shutdown problem that recently affected some users of the iPhone 6s.

"We found that a small number of iPhone 6s devices made in September and October 2015 contained a battery component that was exposed to controlled ambient air longer than it should have been before being assembled into battery packs. As a result, these batteries degrade faster than a normal battery and cause unexpected shutdowns to occur," said a statement posted on Apple's official website.

Expect for the battery malfunctions, the company said they looked for any other factors that could cause an iPhone to shut down unexpectedly. But after intensive investigations, no new factors have been identified.

The company also explained that the iPhone is designed to shut down automatically under extremely cold temperature, aiming to protect the devices electronics from low voltage.

Again, Apple noted that it is not a safety issue.

The statement was released after China's consumer protection watchdog - China Consumer Association (CCA) on Tuesday issued a query letter to ask the company to explain and provide solutions to malfunctions reportedly found in iPhones.

According to the CCA, many consumers continued to complain after Apple announced a free battery replacement program for iPhone 6s users, claiming that the abrupt shutdown problem also exists in iPhone 6, iPhone 6 Plus and iPhone 6s Plus models.

As some consumers have raised questions about the terms and conditions, the CCA has requested Apple to improve customer service for battery replacement, prepare enough components to replace batteries efficiently, and meet basic consumer needs for normal wireless communication.

Apple introduced a free replacement program on Nov 21, to resolve recent reports on the unexpected shutdown of the iPhone 6s.

The official website of the company reads: "This is not a safety issue and only affects devices (iPhone 6s models) within a limited serial number range that were manufactured between September and October 2015."

The company has asked users who have experienced unexpected shutdowns to visit an Apple retail store or an Apple authorized service provider and have their device's serial number checked to confirm eligibility for a free battery replacement.

liuzheng@chinadaily.com.cn