Seoul: With Kim Jong-un turning up the heat with North Korea's fourth nuclear test, South Korea responded on Friday by pumping up the volume. Literally.

At noon on the North Korean leader's birthday, South Korea fired up loudspeakers along the heavily fortified border and resumed the propaganda blasts that brought the reclusive regime to a war footing in August – and then to the negotiating table. South Korea has reinforced defensive positions near the loudspeakers in case of attack, while the North Korean army has stepped up surveillance along the border, the South Korean Defence Ministry said.

A South Korean soldier adjusts equipment used for the propaganda broadcasts at a studio near the border between South Korea and North Korea in Yeoncheon, South Korea, on Friday. Credit:Newsis/AP

The broadcasts challenge the leader's monopoly on information. Speakers were set up at about 10 sites to play messages critical of North Korea's political system along with songs by girl group Apink and folk singer Lee Ae Ran.