iPhone 5 won't deleted previously synced photos

My iPhone 5 will not delete previously synced photos (about 6 GB worth). They were synced from my old computer (a Mac), and remained on my phone after successive syncs when I transferred my iTunes library to my new computer (also a Mac), despite the fact that I did not transfer the photos with my library--in other words, the photos on my phone that were synced from my old computer are no longer on my new computer, but remained on my phone. I have tried both selecting a new folder to sync as well as deselecting the "Sync Photos" checkbox in iTunes (and confirming that I want to delete all my photos), neither of which has worked--in both cases, the synced albums disappear from the photos app, but the Usage page in Settings still indicates that they are taking up the same amount of space, and the phone repeatedly crashes after a few minutes of being on, until eventually all the albums reappear again. I would appreciate any help in solving this matter, as my phone is currently running out of space.