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 said Tuesday, March 15, in an escalation of threats against Seoul and Washington, the Associated Press reports.

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 UN Security Council sanctions in two decades in early March for a nuclear test and long-range rocket launch conducted earlier this year, the AP says.

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

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 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.