Transcript for The volunteer army fighting ISIS in Raqqa, Syria

Tonight we're taking you to the last stronghold of ISIS in Syria where u.s.-backed forces are battling to retake raqqa. The city's fall would be a major defeat for the extremists trying to incite violence around the world, and the stakes couldn't be higher. ABC's Ian Pannell takes us into the streets where the war is raging from building to building and death is always just seconds away. Reporter: One shot, one bullet, one kill. Perched on a rooftop in raqqa, an American man with ISIS in the crosshairs. This isn't really a mission of vengeance. That's a dirty word. But this is more like justice. Reporter: And somewhere on the streets below a fellow fighter, a Syrian woman on a mission to reclaim her homeland. Translator: If there is no determination on the ground, there will be no progress. We're moving forward. With the fighters of the ypg. These are the fighters of the U.S. Coalition working with. They are the foot soldiers. We're moving up to their forward base. Reporter: Two strangers bonded in a battle of the ages against the death cult that is ISIS. What began as a popular uprising against the Assad regime in Syria rapidly descended into a bloody civil war, allowing ISIS to move in. They named raqqa the capital of their caliphate. A darkness descended here, unleashing a frenzy of barbarism that shocked the world. But finally, last month America's allies breached the city walls, giving this volunteer army hope that the fall of ISIS is now within their reach. Our journey to raqqa began hundreds of miles away. We've just left Iraqi territory. We're going over the river. And Syria's on the other side. Personally, this is kind of a big movement. I haven't been able to get back into Syria since 2014. That was partly because of the threat from ISIS. But now their territory's being diminished. The fight is being taken to them. And finally we're able to go back and report on what's going on in the country. Reporter: The Syrian desert is a vast desolate landscape untouched by time where bedouin tribesmen tend their flocks as they've done forever. A land worn down by sand, scrub, and now this war. This land and its people is steeped in tales of sacrifice. And every day the country bears witness to the suffering the war on ISIS has unleashed. They cry "Shaheed." That means martyr. On this day four soldiers were buried in one town alone. Arab and kurd all killed on the fr front line in raqqa against ISIS. These scenes have been repeated day after day across the north of the country. It underlines the amount of sacrifice people are having to make. This community of kurds and Arabs and ethnic Christian groups has done most of the fighting and most of the dying here. The flags of their forces are held aloft as symbols of pride and Independence. But the suffering is real. In the cemetery we saw the mother of a child soldier. Mahmoud had lied about his age to join the war. He was just 13. Childhood ends early here, and the young must learn quickly how to grieve. Neda's just lost her father. Without blinking she vows to take up arms and avenge her father. It's a promise that may not be far off. Sosda Bauer is our guys. For people who've never seen Syria before, where are we and why are we going here? Translator: We're heading to our position on the front line to protect our people from ISIS and to liberate the land. Reporter: She joined an all female fighting force straight out of high school. Around the world women fight for the same rights, the same pay. But in raqqa they fight ISIS. In a community that honors sexual equality sozda leads the band of young kurdish fighters. Translator: You can't capture an entire neighborhood at once. You must go house by house. There are mines and snipers. But we are still advancing. Reporter: There are no vast armies here unlike the fight against ice nis Iraq. And in streets and alleys in raqqa sozda and her troops run the gauntlet of ISIS snipers. We're out of breath here. What the soldier here is saying is because snipers have been targeting the route you have to run in this exact fashion. It's like 200, 250 yards long. It's pretty tiring. And look how lightly armed they are. No tanks, no humvees, not even body armor. The U.S. Is sending some vehicles and guns. I noticed you've got some homemade bombs here. Is that because you're not getting enough supplies, enough weapons from the u.s.-led coalition? Translator: It's true they are sending us weapons, but it's way less than we need for the big battles we are fighting. Reporter: But this is what they're up against. Well-armed ISIS militants in raqqa, filmed in a propaganda video, tried and tested in urban warfare. Sozdar and her volunteer force often have just bravery and belief to protect them. Your morale is high. Because you feel that you're winning against ISIS. On the other side of the city the American sniper known locally as Christian, stalking his prey. It's just harassing fire. They do it to us. We do it to them. It's just like just. It's back and forth. Occasionally we get lucky. We kill each other. Reporter: A volunteer from California, he came here on his own to take on the extremists. How close is ISIS? See the bombed-out building right there? Yeah. Like four, five stories high. Beyond that. Just on the other side? Yeah. Reporter: And this is his motivation. Manchester. Reporter: The names of attacks ISIS has claimed in America and Europe. Orlando, Paris. Reporter: Engraved on his bullets. That just speaks volumes in and of itself. Yeah. What chance did they have? None. So justice for them. Reporter: But the risks here are immense. Putting his life on the line with every step. He'd been fighting in a Christian unit. Seen here in their own propaganda video. ??? No part of you that's scared, nervous, anxious a any point? No. Not really. I came here for this. You know? This is the final stand-up fight between us and them. Reporter: His unit is part of the local forces America's working with, a multiethnic coalition formed to take down ISIS. We have people from Syria. On the right we have Christian here. And then over on the left we have Mesa who's from great Britain. Reporter: Mesa, another volunteer. The fighting is all in this area. Reporter: He's been here fighting for the cause for three years now. Do you ever have those moments where you think what on Earth am I doing here? Those moments of sheer terror. Yes. There's been a few occasions. I've never really doubted the morality of what I'm doing. For me this is very personal. I believe in democracy. And I believe in the people of Syria. Reporter: Fighting to defeat ISIS but also to preserve a religious mix in the Middle East that's facing extinction. What is your message to America? I need just one thing. We need the Christian and American, all people in America believe in the Christian here. Reporter: You need their help? Yeah. Reporter: Without the power of an army behind them the going is as slow as it is dangerous. But there has been progress. Just weeks ago this building was still held by ISIS, a makeshift mosque. You can see here on the wall this is the flag of ISIS here. Here is an extract of poetry. And then if we go through here -- Reporter: Remnants of the religion distorted to fuel their fight. We just have a look here. Sorry, guys. Reporter: And evidence of what it took to keep them going. You can see the needle there. And there are a whole bunch of vials. This is diazepam. It's normally used to calm people down. Also used to treat pain. It's also evidence that ISIS was really up against it here. They felt the need to inject themselves, whether it was to calm themselves down, whether it was to treat pain, but it's a sign of how much pressure they were under. Reporter: Men and women have spent years on the front lines in a war that offers no rest, no respite. I'm not on a mission to like murder anyone. We just defend and we just try to help these people. But if a member of ISIS was in this room right now with all of us, what chance would he give you or you or me? None. Reporter: When we come back, we meet the brides of ISIS and the countless families now caught in the crossfire. And one soldier paying the ultimate price.

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