Reduce the storage space taken up by "System" on my iPhone 6s without restoring and setting up iPhone as new

I have an iPhone 6s with iOS 11 and I have just noticed that a large amount of my iPhone's storage space being occupied by "System" (12.27 GB).

I have tried the answers for the iPhone 4 explained in the question Why is so much "Other" space occupied on an iPhone 4? (except for factory resetting and setting the iPhone up as new as I need to keep my app data) however those did not solve the problem on my iPhone 6s. I have tried restarting and syncing with iTunes, but neither solved the problem.

No need to backup or restore anything ! I just spent some time with a good Apple Care advisors : this problem is caused by iCloud Drive. If you deactivate it, no more problem. It worked for me.