This story was last updated at 10:36 a.m.

By HYUNG-JIN KIM, Associated Press

SEOUL, South Korea (AP) — North Korea caused a new stir Wednesday by publicizing a purported mock-up of a key part of a nuclear warhead, with leader Kim Jong Un saying his country has developed miniaturized atomic bombs that can be placed on missiles.

The North's Rodong Sinmun newspaper carried photos on its front page showing Kim and nuclear scientists standing beside what outside analysts say appears to be a model warhead part — a small, silverish 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 standardized and miniaturized 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 purportednuclear designs, though it remains unclear whether the country has functioning warheads of that size or is simply trying to develop one.

South Korea's Defense Ministry quickly disputed the North's claim that it possesses miniaturized warheads. It called the photos and miniaturization claim an "intolerable direct challenge" to the international community.

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 Monday of pre-emptivenuclear 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.

Karl Dewey, a senior analyst for IHS Jane's, a defense and aerospace publishing company, said the sphere could be a simple implosion weapon, possibly with hydrogen isotopes added to make it more efficient.

"It is unlikely that the object in the photo is a thermonuclear bomb (also referred to as a hydrogen bomb)," he said in a statement. "Thermonuclear weapons are multistage devices and in modern weapons the need to place two separate stages together would result in a more oblong-like structure."

Also shown in the photos is a KN-08 ballistic missile or its model, which reportedly has an estimated range of 10,000 kilometers (6,200 miles), according to South Korean analysts. The KN-08, which North Korea showed off in 2012, is said to be capable of being launched from a road-mobile vehicle, which would make it difficult to monitor via satellite. The South Korean Defense Ministry said it believes the missile hasn't been proven functional.

North Korea says it tested its first H-bomb on Jan. 6, followed last month by the launch of a rocket that put a satellite into orbit but which violated U.N. resolutions because it employs dual-use technology that could also be applied to long-range ballistic missiles.

North Korea's development of smaller nuclear weapons and long-range missiles has long been a matter of concern and could shake up the security balance in Asia.

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Associated Press writers Kim Tong-hyung in Seoul and Eric Talmadge in Tokyo contributed to this report.

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Follow Hyung-jin Kim on Twitter at twitter.com/hyungjin1972