From Easton's 1897 Bible Dictionary :
Resen head of the stream; bridle, one of Nimrod's cities (Gen. 10:12), "between Nineveh and Calah." It has been supposed that the four cities named in this verse were afterwards combined into one under the name of Nineveh (q.v.). Resen was on the east side of the Tigris. It is probably identified with the mound of ruins called Karamless.