SEOUL, South Korea -- North Korea fired two short-range ballistic missiles into the sea on Thursday, South Korea's military said, a likely show of anger at continuing springtime war games by rivals Washington and Seoul and another ratcheting up of hostility on the already anxious Korean Peninsula.
The South Korean Defense Ministry says the missiles were fired from North Hwanghae Province, flew about 310 miles and fell into the water off the country's east coast.
Such missile firings by the North are not uncommon when animosity rises here. North Korea hates the massive annual military drills staged by Seoul and Washington, calling them invasion preparations. The allies call the drills, which this year are described as the biggest ever, defensive and routine. Pyongyang is also angry over tough United Nations sanctions following its recent nuclear test and long range rocket launch.
The firings come a day after North Korea caused a new stir by publicizing a purported mock-up of a key part of a nuclear warhead, with leader Kim Jong Un repeating a claim that 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 purported nuclear designs, though it remains unclear whether the country has functioning warheads of that size or is simply trying to develop one.
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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.
U.S. State Department spokesman John Kirby declined to comment on North Korea's nuclear capabilities, saying it was an intelligence matter, but he told reporters the U.S. takes Pyongyang's rhetoric seriously. He said the North Korean leader, who is thought to be in his early 30s, needs to pay more attention to the needs of his people rather pursue "reckless capabilities."
North Korea warned Monday of pre-emptive nuclear strikes after the United States and South Korea began the 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.
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.
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 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 Hydrogen 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.