Kim Jong-un says North Korea now possesses nuclear warheads small enough to fit on ballistic missiles.
The hermit kingdom made the claim by publishing photos on the front page of its ruling party newspaper that show the country's leader standing beside what it said was miniaturised nuclear warhead during a meeting with his top scientists.
The Rodong Sinmun newspaper for the Workers' Party said Kim met his nuclear scientists for a briefing on the status of their work and declared he was greatly pleased that warheads had been standardized and miniaturized for use on ballistic missiles.
The party newspaper photos showed Kim and the scientists standing by what outside analysts say appears to be the model warhead - a small, silverish globe presented on a low table in a hangar with a ballistic missile or a model ballistic missile in the background.
The North claims it tested its first H-bomb in the 6 January nuclear test, which was followed last month by the launch of a rocket that put a satellite into orbit but which is seen as a violation of U.N. resolutions because it contains dual-use technology that could also be applied to long-range ballistic missiles.
Its development of smaller nuclear weapons and long-range missiles that could be used to deliver them to targets overseas has long been a matter of concern and could potentially shake up the security balance in Asia.
If the North succeeds in developing a credible warhead and missile, it would most deeply impact the United States, South Korea and Japan, but Russia and China, which are friendlier to the North, have strongly denounced its nuclear program.
South Korea's Defense Ministry has said it is analysing the objects shown in the North Korean photos.
Pyongyang has previously said it has nuclear warheads small enough to put on long-range missiles, but experts have questioned such claims.
This was the first time the North has publicly portrayed what its designs look like, though it remains unclear whether the North has a functioning warhead of that size or if it is simply trying to develop one.
On Monday, North Korea warned of pre-emptive nuclear strikes after the United States and South Korea began holding their biggest-ever war games, which will go on until the end of April. Tensions have been high after North Korea's recent nuclear test and rocket launch, which prompted the United Nations to adopt tough new sanctions.