The source, who requested anonymity, said the drills simulated bombarding Seoul with artillery shells loaded with concrete instead of live ammunition.
In the last few months, North Korea's military has conducted several airborne infiltration exercises with fighter jets and cargo aircraft.
"An analysis of the North Korean military's winter training showed that live-fire artillery drills and airborne infiltrations increased," the source said. "Overall, its winter training has increased aggression."
The move comes after the North carried out its third nuclear test earlier this month.
Pyongyang has hinted it could carry out more nuclear and missile tests if the United Nations Security Council tightens sanctions.