North Korean leader Kim Jong Un has warned of impending tests of a nuclear warhead explosion and ballistic missiles capable of carrying atomic warheads, state media reported Tuesday, in an escalation of threats against Seoul and Washington.

Kim issued the order for the tests "in a short time," according to the Korean Central News Agency. The KCNA dispatch did not say if Kim gave specific dates for the tests.

The announcement comes as North Korea said it had mastered a key remaining technology needed to develop a reliable long-range missile capable of striking the U.S. mainland.

It is unclear if the tests would happen soon, given that any tests would likely invite harsher international sanctions after the country was hit by the toughest U.N. Security Council sanctions in two decades in early March for a nuclear test and long-range rocket launch conducted earlier this year.

In the past, North Korea has typically conducted nuclear tests and rocket launches every three to four years.

Kim's threats came as his country furiously reacts to ongoing annual military drills by Seoul and Washington, which Pyongyang views as an invasion rehearsal.

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.

He made the comments while guiding a successful simulated test of a re-entry vehicle, which is needed to return a warhead safely back into the Earth's atmosphere from a long-range missile launch.

Information from secretive, authoritarian North Korea is often impossible to confirm and there is virtually no way to check how genuine its claims are on developing re-entry vehicle technology.

South Korean defense officials and many outside experts have said the North does not yet have a workable re-entry vehicle, meaning the country does not have a reliable missile capable of hitting the U.S. mainland.

Pyongyang often makes fiery warlike rhetoric in times of tension with the outside world.