China Consumer Association earlier last week sent a letter to Apple, requesting that it clarify the reason, state-run Xinhua news agency reported.
Consumers across China have filed complaints on the country's microblog service Weibo, claiming their mobile phones unexpectedly switch off despite displaying battery levels over 50 per cent.
Stressing the issue is not safety-related, Apple China said in a statement that the unexpected power-off only affects handsets with certain serial numbers. It agreed to replace the batteries of faulty iPhone 6s manufactured between September and October 2015 free of charge.
Other models with the same problem are not included in the current replacement plan. Consumers can check whether they qualify for free battery replacement at any Apple store or authorised reseller, the company added.