There's more to do in South Korea's heavily forested Rocket Valley, just a few kilometers from the demilitarized zone separating the two Koreas, than fire rockets. In quieter times, people tend vegetable patches along ice-cold streams.

But last week, a U.S. artillery brigade based in the South heated things up, launching a barrage of rockets close to the border town of Cheorwon.

The live-fire drills came hours after a report by reclusive North Korea that it had miniaturized nuclear warheads to be mounted on ballistic missiles and leader Kim Jong Un had ordered further improvements to its arsenal.