SHANGHAI (Reuters) - A Chinese consumer group has asked Apple <AAPL.O> for information about the slowing of older iPhones after operating system updates, demanding a reply before Friday, state news agency Xinhua reported.

The query from the Shanghai Consumer Council came in response to consumer feedback that old iPhones became sluggish after upgrading the software to iOS 10.2.1, it said.

In a letter to Apple on Monday, the council requested an explanation for the slow-down and information about what Apple planned to do to rectify the problem, Xinhua reported.

The California-based company acknowledged in December that iPhone software can slow down some phones with battery problems and apologised for the issue. It also cut battery replacement costs and said it would change its software to show users whether their phone batteries were working well.

The Shanghai Consumer Council, a non-government organisation approved by the Chinese authorities, said it had received 2,615 complaints about Apple products and services in 2017, compared with 964 complaints in 2015.

(Reporting by John Ruwitch; Editing by Eric Meijer)