A medium-range ballistic missile fired Wednesday by North Korea flew about 1,000 kilometres (620 miles) and landed near Japan's territorial waters, Seoul and Tokyo officials said, one of the longest flights by a North Korean missile.

The U.S. Strategic Command said North Korea fired two presumed Rodong missiles simultaneously, not just one. The command said initial indications were that one of the missiles exploded immediately after launch, while the second was tracked over North Korea and into the Sea of Japan.

North Korea has recently claimed a series of technical breakthroughs in its goal of developing a long-range nuclear missile capable of reaching the continental U.S. South Korean defence officials say North Korea doesn't yet have such a weapon, but some civilian experts believe the North has the technology to mount warheads on shorter-range Rodong and Scud missiles that can strike South Korea and Japan.

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According to the South Korean and Japanese announcements, one suspected Rodong missile lifted off from the North's western Hwanghae province and flew across the country before falling in waters between the Korean Peninsula and Japan. Seoul's Joint Chiefs of Staff said in a statement that it "strongly condemns" the missile launch because it explicitly shows the North's intentions of being able to launch missile attacks on South Korea and neighbouring countries.

Japan's Defence Ministry said the missile landed inside Japan's exclusive economic zone, the 200-nautical mile offshore area where a nation has sovereign rights for exploring and exploiting resources. Japanese media reported it was the first North Korean missile that has splashed down in Japan's EEZ.

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"It imposes a serious threat to Japan's security and it is an unforgivable act of violence toward Japan's security," Prime Minister Shinzo Abe said.

At the United Nations, the Security Council scheduled an emergency meeting Wednesday afternoon in response to a request by the United States and Japan.

The United States condemned the launches as violating U.N. Security Council resolutions that ban North Korea's use of ballistic missile technology.

"This provocation only serves to increase the international community's resolve to counter (North Korea's) prohibited activities, including through implementing existing U.N. Security Council sanctions," said Navy Cmdr. Gary Ross, a Pentagon spokesman.

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NATO Secretary-General Jens Stoltenberg also condemned the launches, saying North Korea should "immediately cease and abandon all its existing nuclear and ballistic missile activities in a complete, verifiable and irreversible manner" and "refrain from any further provocative actions."

North Korea has previously fired Rodong and other missiles into the sea, but South Korean analysts say Wednesday's 1,000-kilometre flight was one of the longest for a North Korean test.

Several other North Korean rockets have gone farther and even over Japan. But North Korea called them satellite launches while Washington, Seoul and Tokyo said they were disguised tests of missile technology. After several failures, the North put its first satellite into space aboard a long-range rocket in December 2012, and conducted another successful satellite launch in February.

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In June, North Korea, after a string of failures, sent another type of mid-range missile known as Musudan more than 1,400 kilometres (870 miles) high. Analysts said the high-altitude flight meant North Korea had made progress in its push to be able to strike U.S. forces throughout the region.

North Korea routinely conducts weapons tests, but the latest launch came after North Korea warned of unspecified "physical counter-actions" against a U.S. plan to deploy an advanced missile defence system in South Korea by the end of next year.

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On July 19, North Korea fired three ballistic missiles into the sea, according to Seoul defence officials. The North's state media later confirmed that it fired ballistic rockets carrying trigger devices for nuclear warheads as part of simulated pre-emptive atomic attacks on South Korea.

Analyst Kim Dong-yub at Seoul's Institute for Far East Studies said the latest Rodong launch appeared to be aimed at showing an ability to attack U.S. military bases in Japan, a major source of reinforcements for U.S. troops should a war break out on the Korean Peninsula.

Hyon Kwang Il, director of the scientific research department at the North's National Aerospace Development Administration, told The Associated Press last week that Pyongyang's military had mastered "the main and core techniques in long-range ballistic missile technology, including the starter engine technology, how to separate the different stages, guidance control techniques and re-entry and it's all ready to be operational."

North Korea is known to have an arsenal of estimated 300 Rodong missiles whose maximum range is 1,300 kilometres (800 miles). A Rodong fired in March flew about 800 kilometres (500 miles) while two other Rodongs launched in 2014 flew about 650 kilometres (400 miles).

North Korea is expected to carry out more weapons launches in coming weeks to protest annual U.S.-South Korean military drills that begin later this month. North Korea describes the drills as an invasion rehearsal.

The Korean Peninsula remains in a technical state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty. About 28,500 U.S. troops are stationed in South Korea and tens of thousands of more in Japan.