North Korea
North KoreaReuters

North Korean leader Kim Jong-un called for more nuclear tests to improve the isolated nation's nuclear capability in the face of joint military drills between South Korea and the United States, whom he described as "enemies." North Korea is facing toughest set of sanctions from the United Nations after it conducted a nuclear test in January. 

Kim met nuclear scientists and technicians and supervised a ballistic missile launch this week. Earlier this week, the North Korean leader claimed the country had miniaturised nuclear warheads that could be mounted on ballistic missiles.

"Dear comrade Kim Jong Un said work ... must be strengthened to improve nuclear attack capability and issued combat tasks to continue nuclear explosion tests to assess the power of newly developed nuclear warheads and tests to improve nuclear attack capability," the state media Korean Central News Agency reported Friday. 

North Korea Thursday fired two short-range ballistic missiles into the Sea of Japan; it was reportedly an exercise to check the working of "exploding nuclear warheads from the preset altitude above targets in the ports under enemy control."

The firing of missiles and aggressive comments from Kim have increased in recent days as the leader directed scathing verbal attacks on the U.S.-South Korea joint exercise. 

North Korea had issued a warning earlier this week, threatening to launch "pre-emptive nuclear strike of justice" if the U.S. and South Korea persisted with the military drill. However, the two allies carried out their military exercises with the largest participation of troops between them, including 17,000 U.S. troops and 300,000 South Korean troops. This year's drill was reportedly focused on the capability to launch strikes targeting the top leadership of the Kim Jong-un regime.