Hey there Joe:
Some UFO reports have definitely been attributed to secret USAF aircraft. For example we know the SR-71 was responsible for some, as were the Skyhook balloons.
As has been alluded to however, the radical maneuvering of some UFOs precludes any known aircraft then or now. A whole new propulsion sytem needs to be developed. So long as we're using some sort of internal combustion or even nuclear rocket technology, we're not going to be able to pull off instant stops, instant accelleration to personic speeds and instant sharp angled high G-force turns.