North Korean leader Kim Jong Un inspected the loading of a hydrogen bomb into a new intercontinental ballistic missile, Pyongyang’s state media said Sunday, a claim to technological mastery that some outside experts will doubt but that will raise already high worries on the Korean Peninsula.

As part of North Korea’s work to build an arsenal of viable nuclear ICBMs that can reach the U.S. mainland, Kim inspected during a visit to the North’s Nuclear Weapons Institute the loading of a “homemade” H-bomb into an ICBM.

The explosive power of the bomb is adjustable from tens kiloton to hundreds kiloton,” the state run Korean Central News Agency said.

North Korea in July conducted its first ever ICBM tests, part of a stunning jump in progress for the country’s nuclear and missile program since Kim rose to power following his father’s death in late 2011.

The North conducted two nuclear tests last year alone. The first involved what it claims to have been a hydrogen bomb and the second was its most powerful ever. Experts and outside governments are skeptical of the hydrogen claim, but it is almost impossible to independently confirm North Korean statements about its highly secret weapons program.

The key question is how far North Korea has gotten in efforts to consistently shrink down nuclear warheads so they can fit on long-range missiles.

South Korea’s main spy agency has previously asserted that it does not think Pyongyang currently has the ability to develop miniaturized nuclear weapons that can be mounted on ballistic missiles. Some experts think the North may have mastered this technology.

North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to eventually carry smaller versions of those bombs.

(AP)