...these are a response to the Attacks during Balance of Terror. They were to patrol the neutral zone while the destroyed outposts were replaced with armed installations and to then provide support to those installations.
You can see more images of this design on Uyeno's blog, which also features lots of other designs. Here's one of his ships, the USS Mughi, from this year's edition of the Ships of the Line calendar, in motion!:
Trek BBS user toughlittleship meanwhile pointed out the DS9 images appears to be an alternate take on the cover for Plagues of Night.