Apple has admitted that some iPhone 6S devices can suddenly shut down for no apparent reason.

The tech giant is offering owners of the problematic smartphones free replacement batteries.

"A very small number" of iPhone 6S devices manufactured between September and October 2015 are affected, Apple (AAPL) said in a statement Monday. It didn't provide details on the battery fault that is causing the phones to unexpectedly go dark.

Related: Apple will fix iPhone 6 Plus devices with 'Touch Disease'

Apple's announcement comes less than a week after a consumer watch group in China launched an investigation into the problem.

"A considerable number" of Chinese users reported their iPhone 6 and 6S devices would suddenly turn off despite showing plenty of battery life, the China Consumers Association said in a statement. The phones would not turn back on even when plugged into chargers, according to the consumer group.

Apple said the battery problem is "not a safety issue." Its big rival Samsung pulled its flagship Galaxy Note 7 smartphone after repeated reports of the devices bursting into flames. Samsung initially said batteries in some Note 7 phones were to blame but is yet to fully explain the cause.

Related: More than 500 South Koreans sue Samsung over botched Note 7 recall

Apple's battery replacement offer for the iPhone 6S applies worldwide. Owners of eligible phones with cracked screens will have to repair them before getting the new battery, Apple said.

Consumers were plagued by a similar iPhone problem earlier this year. A software glitch with the iPhone 6S and 6S Plus caused the battery icon to display more battery life than was actually left, leaving many people frustrated when their phones appeared to suddenly shut down. The company fixed the problem with an operating system update.

And just last week, Apple acknowledged iPhone 6 Plus devices suffer from "Touch Disease" -- when the phone's touchscreen stops working after it has been bent or repeatedly dropped on the floor.

The tech company rolled out a repair program, charging customers $149 to cure the disease.