relationship in syria, and it's up to the russians to explain what they're up to. >> reporter: mod ullr units have been installed. a defense official tells cnn, and this image broadcast on syrian state television, identified by analysts as a modern russian-made armored vehicle, not previously seen in syria, and painted in russian army camouflage. what the russians might or might not be doing at the moment, that's of concern to secretary kerry, and who would replace aassad? >> reporter: syria is russia's key alou. moscow also has a strategic naval base. if the fragile regime of bashiral assad falls, so too may russian influence in the region. russia and the united states had been talking about finding a political solution to the crisis. administration officials until recently expressing hope that moscow was open to a future without assad.