Kim Jong Un has been pictured posing next to a a mock-up of a miniaturised nuclear warhead.
The dictator appeared in images which, according to North Korean state media, showed him standing beside what outside analysts say appears to be a model warhead part - a small, silver globe with a ballistic missile or a model ballistic missile in the background.
The newspaper 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 standardised and miniaturised for use on ballistic missiles.
Information from secretive, authoritarian North Korea is often impossible to confirm and the country's state media have a history of photo manipulations.
But it was the first time the North has publicly displayed its purported nuclear designs, though it remains unclear whether the country has functioning warheads of that size or is simply trying to develop one, the Associated Press reported.
South Korea's Defence Ministry quickly disputed the North's claim that it possesses miniaturised warheads. It called the photos and miniaturisation claim an "intolerable direct challenge" to the international community.
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The photos come amid heightened tensions after the United Nations imposed harsh sanctions on North Korea for its nuclear test and long-range rocket launch earlier this year.
North Korea warned on Monday of pre-emptive nuclear strikes after the United States and South Korea began their biggest-ever war games, which are to continue until the end of April.
North Korea has previously said it has nuclear warheads small enough to put on long-range missiles capable of striking the U.S. mainland, but experts have questioned those claims.
The round object shown in the photos appears to be a model of a warhead trigger device which would contain uranium or plutonium, according to nuclear expert Whang Joo-ho of Kyung Hee University in South Korea. He said it was obviously a model because Kim and others would not stand near an actual device because of concerns about radioactivity.