The push into Qaryatain took place under the cover of Russian airstrikes. It comes days after IS was pushed out of the nearby ancient city of Palmyra.
Qaryatain used to be home to a sizeable Christian population. The 1500-year-old Catholic monastery of Mar Elian has not been spared from the destruction.
IS captured the town in August last year. It abducted hundreds of residents, including dozens of Christians. many were later freed.