The high-tech aircraft joins two other US spy planes already in the skies above the hermit state.
The Civil and Military Air Twitter feed posted this evening: “Cobra Ball – Ballistic Missile detection platform airborne. No position showing.”
The launch of the aircraft comes just days before Donald Trump is due to visit Seoul as part of an East Asia tour.
The South Korean intelligence agency said today: “There is a possibility of a new missile launch given the active movement of vehicles around the missile research institute in Pyongyang.
“The North will constantly push for further nuclear tests going forward, and the miniaturisation and diversification of warheads.”
According to the US Air Force website, the RC-135S Cobra Ball is designed to collect visual and electronic data on ballistic targets.
American commanders have just three of the high-tech planes at their disposal and it is kitted out with a sophisticated range of sensors and recording equipment.
The huge aircraft is deployed with at least 10 specialist crew members, including electronic warfare officers.
More to follow…