North Korea said it test-fired ballistic rockets as part of a simulated pre-emptive attack on ports and airfields in South Korea.
Leader Kim Jong Un guided the launches and expressed his satisfaction with them, the North's official Korean Central News Agency reported, without saying when the tests happened.
However on Tuesday South Korea said the North fired three ballistic missiles from inland that flew about 310 to 375 miles before crashing into the sea.
The KCNA said the launches "once again examined the operational features of the detonating devices of nuclear warheads mounted on the ballistic rockets at the designated altitude over the target area".
Lee Chun-geun, a scientist at South Korea's Science and Technology Policy Institute, said the North Korean missiles were believed to be carrying warheads, which contain trigger devices but not plutonium or uranium, to see whether those warheads could detonate properly.
The launches were the latest in a series of weapons tests North Korea has carried out since March.
Pyongyang said it conducted a test of a warhead-detonating device when it fired a submarine-launched missile in April.
North Korea is pushing to manufacture a warhead small enough to be placed on a long-range missile that can reach the continental US, but South Korean defence officials say the North does not yet have such a miniaturised warhead.
Some civilian experts, however, believe the North has the technology to put warheads on shorter-range missiles that can strike South Korea and Japan.
The three reported launches came as North Korea criticised the planned deployment of an advanced US missile defence system in South Korea.