North Korean leader Kim Jong Un said the country has miniaturized nuclear warheads to be mounted on ballistic missiles, the North's KCNA news agency reported on Wednesday.

"The nuclear warheads have been standardized to be fit for ballistic missiles by miniaturizing them," KCNA quoted him as saying as he inspected the work of nuclear workers, adding "this can be called true nuclear deterrent."

The comments were Kim's first direct mention of the claim previously made repeatedly in the country's state media to have successfully miniaturized a nuclear warhead to be mounted on a ballistic missile, which is widely questioned.

Kim also inspected the nuclear warheads designed for thermo-nuclear reaction, KCNA said, referring to a hydrogen bomb that the country claimed to have tested in January.

North Korea conducted its fourth nuclear test on Jan. 6 and claimed it was a successful hydrogen bomb test, which was disputed by many experts and the governments of South Korea and the United States.

The U.N. Security Council imposed harsh new sanctions on the isolated state last week for the nuclear test, and Pyongyang has stepped up its belligerent rhetoric through state media.

Last week Kim ordered his country to be ready to use nuclear weapons at any time in the face of growing threats from enemies.