BEIRUT, Dec 21 - The U.S.-backed Syrian Democratic Forces would have to withdraw fighters from the battle against the militant group Islamic State to protect its borders in the event of a Turkish attack, spokesman Mustafa Bali told Reuters on Friday. 'Fighting terrorism will be difficult because our forces will be forced to withdraw from the Deir el-Zor front to take up positions on the border with Turkey to stop an eventual attack,' Elham Ahmed, the co-chair of the SDF's political wing, said in Paris.