A day after claiming that it could wipe out Manhattan by dropping a hydrogen on New York City, North Korean leader Kim Jong Un ratcheted his threats as he warned of impending tests of a nuclear warhead explosion and ballistic missiles capable of carrying nuclear warheads, state media reported on Tuesday.

Kim issued the order for the tests "in a short time," according to the Korean Central News Agency (KCNA) as reported Fox News. No specific date was mentioned.

North Korea earlier boasted that it has already "mastered" the key technology needed to develop a reliable long-range missile capable of striking the U.S. mainland.

Kim said "a nuclear warhead explosion test and a test-fire of several kinds of ballistic rockets able to carry nuclear warheads will be conducted in a short time to further enhance the reliance of nuclear attack capability," according to KCNA.

The North Korean leader issued the announcement while guiding a successful simulated test of a re-entry vehicle that could be armed with a nuclear warhead. The safe re-entry into the Earth's atmosphere of a long-range missile is a vital component of a ballistic missile system.

South Korean defence officials and many outside experts have said that North Korea does not have a workable re-entry vehicle yet, which means that it still does not have a reliable missile system that could hit the U.S. mainland.

However, although Kim may just be exaggerating his military's technical capabilities, the near-daily drumbeat of boasts and warnings from North Korea underlines its anger at efforts to thwart its ambitions, according to the Washington Post.

On Sunday, Pyongyang claimed it has a hydrogen bomb capable of destroying New York City. "Our hydrogen bomb is much bigger than the one developed by the Soviet Union," DPRK Today, a state-run outlet, reported.

"If this H-bomb were to be mounted on an intercontinental ballistic missile and fall on Manhattan in New York City, all the people there would be killed immediately and the city would burn down to ashes," the report said, citing a nuclear scientist named Cho Hyong Il.

Earlier on Saturday, joint U.S. and South Korean forces conducted a big military exercise, storming a simulated North Korean beach defence, Fox News reported.

The amphibious landing drills on South Korea's east coast are part of eight weeks of exercises with the U.S., which the South has said are the largest ever.

North Korea called the exercises "nuclear war moves" and threatened to respond with attacks, according to Reuters.