u.s. backed fighters report fierce resistance as they attack the last isis enclave in syria. they launched an operation on saturday. it is a small valley town near the iraqi border. and it may also hold some of the most battle hardened isis fighters determined to fight to the death. ben, you did have to pull back from the front lines there. talk to us about what's been happening in the past few hours. >> reporter: rosemary, we woke up to the sound of heavy machine gunfire just down the road this morning where we were in our previous position, which was about a kilometer and a half from the front line. we went up to the rooftop of our building where we've been doing