combatting isis are ongoing. they have been complicated, though, by rising tension with russia. >> cnn's frederick pleitgen got exclusive access to u.s. flight crew on a mission over syria. you have to take a look at this. >> reporter: faced with new russian threats, the u.s. military not backing down in the skies over iraq and syria. we're on a kc-10 tanker plane, refueling the fighters pounding isis. of course there's some tension since the russians have announced they don't want to communicate with the u.s. anymore, in the skies over syria. that's why crews like this one take great care when they fly into syrian air space. >> reporter: stopping the communications significantly increases the ruisk of mid-air collisions over this crowded air space where u.s. coalition and russian planes operate very close to one another. russia made the move after russia hit a syrian airfield with cruise missiles last week, in response to a chemical attack