Transcript for Battle to take back city of Raqqa, the last ISIS stronghold in Syria

take evasive action to prevent collision. On the front lines in Syria, witnessing a last stand for ISIS in raqqah, and tonight you will see right here what our team discovered, what ISIS was using to keep going in their fight. ABC's Ian Pannell from Syria. Let's go. Reporter: Tonight, on the run in raqqah. If you want to get to the front line here, there's just one way. Run and zigzag to avoid ISIS snipers. We're moving forward. Progress is slow and dangerous. We move to the old city where the war against ISIS rages. Some 3,000 militants trying to hang onto raqqah, their de facto capital and last major stronghold. They have U.S. Forces taking fire, and you hear the crackle of gunfire every few minutes. And here right on the front line, a volunteer from California who has come here on his own as a sniper. This isn't a mission of vengean vengeance. That's a did urrty word, but this is more like justice, you know? Reporter: And he has bullets to prove it. Engraved in terrorist attacks in America and Europe. You know. What chance did they have, you know? None. So justice for them. Reporter: He takes us to a makeshift mosque set up by ISIS. This is the flag of ISIS here. Here we found evidence of drug abuse. You can see the needle there. A sign of the pressure the militants now face and how fiercely they fight. In the battle some now call the last stand of ISIS. For the going is slow, the battle could last for months, and the kind of attacks the American sniper says he is here to stop may never end. Thank you all. With we turn to the health care showdown. It's coming tomorrow.

This transcript has been automatically generated and may not be 100% accurate.