Kim Jong-un has said North Korea will soon conduct tests of nuclear warheads and ballistic missiles.
He said the tests would "enhance the reliance" of the North's nuclear attack capability, state media reported.
The North has issued a series of increasingly bellicose threats in the wake of sanctions imposed after nuclear and missile tests earlier this year.
But though it is known to have nuclear weapons, many analysts doubt North Korea has the ability to deliver them.
Last week, Mr Kim claimed his scientists had developed nuclear warheads small enough to fit on ballistic missiles. The claims were accompanied by pictures of the leader standing alongside what state media said was a miniaturised nuclear warhead.
South Korea's defence ministry was sceptical about the claim, saying it thought the North had "not yet secured miniaturised nuclear warheads" and that it believes the North "hasn't acquired re-entry technology".
On Tuesday, President Park Geun-hye warned that if the North continues "its provocations and confrontation with the international community and does not walk the path of change, it will walk the path of self-destruction".
Kim Jong-un's latest comments came as he was overseeing a simulation for the kind of re-entry technology needed to bring a nuclear-armed ballistic missile back into the atmosphere, the KCNA news agency reported.
It quoted him as saying the tests would take place "in a short time" but did not mentioned a timescale.
In January, the North conducted its fourth nuclear test, which it claimed was of a powerful hydrogen bomb. This was followed in February by the launch of a satellite using banned missile technology. Both were in contravention of existing UN sanctions.
In the past week, US and South Korean forces have been holding their largest ever annual joint military exercises, known as Key Resolve and Foal Eagle.
The drills routinely generate tension, but this year Pyongyang threatened to launch a "pre-emptive nuclear strike of justice" against the US and South Korea.
Some analysts believe that the latest string of threats will add to speculation that the North could be preparing for yet another nuclear test.