North Korea Wednesday caused a new stir by publicizing a purported mock-up of a nuclear warhead for the first time, with leader Kim Jong Un saying his country has developed miniaturized atomic bombs to be placed on missiles.

The North's Rodong Sinmun newspaper carried photos on its front page that showed Kim and nuclear scientists standing beside 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.

It 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.

The disclosure comes amid heightened tensions in the wake of harsh U.N. sanctions on North Korea for its nuclear test and long-range rocket launch earlier this year. North Korea warned Monday 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.

The round object shown in the photos appears to be a model of a trigger device for a warhead, which would contain uranium or plutonium inside it, according to nuclear expert Whang Joo-ho of Kyung Hee University in South Korea Whang said it was impossible to judge from the photos if North Korea has mastered the miniaturization technology because it was not known if the object was real or not. But he said its shape looks similar to ones used in existing nuclear warheads developed by other countries.