How can I delete hidden photos from my iPhone 6?

I purchased the iPhone 6 64GB and have had some difficulties regarding the photo library. Whenever I sync my iPhone with iTunes on my MacBook Pro, it says that I have 6,343 photos on my phone, consuming 21GB of storage. However, on the iPhone itself, it says there is only 1,432 in the Camera Roll and 1,047 in Recently Deleted. Even emptying my Recently Deleted folder would leave me with 3,864 photos that are not visible on my iPhone. I back-up my iPhone to iTunes/iPhoto on my MacBook and have ensured that there are no duplicate photos. I have also tried deleting attachments in the Messages app, and that has not made a difference either. I would really like to find out how to delete these mystery photos to free up some space on my phone. Any help is greatly appreciated.