After secretly test flying a Pakistan Air Force F-6 in 1965, the United States concluded that the J-6 and MiG-19 were more formidable fighter aircraft than the more modern and powerful MiG-21 'Fishbed', as well as the older MiG-17 'Fresco' and its Chinese version, the J-5. The supersonic speed advantage provided by the MiG-21's more modern turbojet engine was found to be not as useful in combat as originally thought, because aerial dogfights at the time were conducted almost entirely in the sub-sonic speed regime. The J-6 (and hence the MiG-19 also) was found to be more manoeuvrable than the MiG-21 and, although slower, its acceleration during dogfights was considered adequate. The North Vietnamese Air Force fielded at least one unit of J-6 during the war, the 925th Fighter Regiment, beginning in 1969.[6]