In the balloon warfare between North Korea and South Korean activists, one Seoul-based group has honed its tech expertise to develop balloons capable of dispersing leaflets and electronic speakers hundreds of kilometres across the border.

South Korean forces fired warning shots on Sunday after North Korean troops accidentally crossed their shared border, Seoul’s military said, as tensions simmer on the Korean Peninsula following a series of tit-for-tat exchanges between the two sides.

For two hours on Sunday, a South Korean loudspeaker near the two countries’ border played a local radio program that included news and K-pop.