Technically, the North said the test was aimed at further developing the miniaturisation of nuclear warheads so they could be mounted on ballistic missiles.
In its statement the North said it could now produce "at will, and as many as it wants, a variety of smaller, lighter and diversified nuclear warheads of higher strike power".
In recent months, the North has conducted a series of ballistic missile launches and has in the past often stated its aim of hitting US targets.
The North has previously made claims on "miniaturised" nuclear warheads but they have never been independently confirmed.