Syrian President Bashar Assad is threatening to attack a region held by U.S.-backed Kurdish fighters in northeastern Syria if talks fail to bring the area back under Damascus' authority.

Assad said in an interview with Russia Today television which aired on Thursday that the U.S. troops, who operate air bases and outposts in the Kurdish-administered region, will have to leave country.

He says he has opened the door to negotiations with the Kurdish-run administration while also preparing to "liberate by force."

Forces loyal to Assad, who is backed by Russia and Iran, and the Syrian Kurds have clashed sporadically over the eastern oil province of Deir el-Zour. They led rival fronts against Islamic State militants last year, and they maintain a protracted front against each other along the Euphrates River.