Flying East at FL370 about 200nm West of ANC and 50-100 south of SPARREVOHN. Observed what first appeared to be aircraft 2-10nm north of us and flying parallel in same direction appx. 2,000/4,000ft below our altitude. Observed several lights on craft (nav lights?) and one white strobe on aft side of craft. Asked ATC about traffic and they reported "no traffic" in our area except a King Air 100nm north and at FL260 which was too far away to be this craft which was keep ing pace with us (500kts ground speed which King Air is un-capable of). We slowly moved ahead of craft and eventually (2-4min) lost sight at our 7 o'clock position when be began descent into ANC.