SEOUL, Korea, Republic Of - North Korea responded to South Korean unilateral sanctions Thursday by firing short-range ballistic missiles into the sea in a show of defiance and vowing to "liquidate" all remaining South Korean assets at former co-operative projects in the North.

The moves are the latest in an escalating standoff between the Koreas that began in January when North Korea detonated what it said was an "H-bomb of justice," its fourth nuclear test. Since then, Pyongyang has launched a long-range rocket; Seoul has shut down the last remaining co-operative project between the rivals, a jointly run factory park in the North Korean border town of Kaesong, and slapped sanctions on the North over its recent nuclear test and rocket launch; the UN has imposed sanctions; and the North has threatened nuclear strikes on Seoul and the U.S. mainland.

The missile firing Thursday comes a day after the North released photos of leader Kim Jong Un standing beside what appears to be a nuclear warhead mock-up.

The North's Committee for the Peaceful Reunification of Korea said Thursday that Pyongyang will "liquidate" South Korean assets at the closed Kaesong factory park and the scrapped tourism resort at Diamond Mountain, both of which are in North Korea. In a continuation of bellicose rhetoric that has spiked in recent weeks, the statement said North Korea will also take a series of unspecified steps to impose "lethal" military, political and economic blows on the South Korean government to accelerate its "pitiable demise."

North Korea didn't say what exactly it will do with the South Korean assets. But North Korea observers said the country could move the remaining manufacturing equipment at Kaesong to other industrial areas or convert them for military use while using South Korean-owned facilities at Diamond Mountain for its own tourism project.

At Kaesong, the South Korean assets include some buildings, manufacturing equipment and finished products. In the mountain resort, South Korean assets are a hotel, a spa, a golf course and a building used for on-and-off reunions of Korean families separated by war.

The North Korean statement called South Korea's unilateral sanctions "laughable, unsightly" behaviour, and referred to South Korea's female president, Park Geun-hye, as an "American prostitute," the latest in a series of crude sexist attacks on her. The insult appeared only in a Korean-language dispatch as its English version called Park a "servant of the U.S."

All of this ratchets up hostility on the already anxious Korean Peninsula. But, so far, analysts don't see the possibility of things spinning out of control. In part, some of the North's rhetoric and actions is intended for domestic audiences to show government strength ahead of a major meeting of the ruling party in May.

The South Korean Defence Ministry says the North's missiles were fired from North Hwanghae province, flew about 500 kilometres and fell into the water off the country's east coast. They are believed to be Scud-type missiles, said ministry spokesman Moon Sang Gyun.

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 say the drills, which this year are described as the biggest ever, are defensive and routine. Pyongyang is also angry over tough United Nations sanctions following its recent nuclear test and long-range rocket launch.

North Korea on Wednesday 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 was briefed by his nuclear scientists 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.

South Korea's Defence 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 told reporters the U.S. takes Pyongyang's rhetoric seriously.

North Korea warned Monday of pre-emptive nuclear strikes after the United States and South Korea began the war games, which are to last 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.

Also shown in the photos is a KN-08 ballistic missile or its model, which reportedly has an estimated range of 10,000 kilometres, 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 Defence 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 UN resolutions because it employs dual-use technology that could also be applied to long-range ballistic missiles.

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AP writer Kim Tong-hyung contributed to this report from Seoul.