With a simulated air assault and a subsequent `humanitarian relief operation` on Monday, the Cooperative Key 2001 exercise in Southern Bulgaria entered a decisive stage. The multinational force provided an air swath for an `assault` on a `target territory` (the Ouzoundjovo airfield). A-10 planes of the US Air Force took part in the air space clearing operation. Troops transported in C-130 Hercules planes of the Romanian and French air forces performed a parachute landing at the Ouzoundjovo airfield, which had been `occupied` by the `enemy.` They took the airfield, rescued the `POWs` and transported the `wounded` to a field hospital near the Kroumovo Air Base, BTA reported.