Five of those launches failed, many exploding in midair or crashing, and the sixth flew only about 400 kilometers (250 miles), South Korea's Joint Chiefs of Staff said, an improvement but still well short of the missile's potential 3,500-kilometer range and not long enough to be classified as intermediate.
Despite the repeated failures, the North's determination in testing the Musudan worries Washington and its allies, Tokyo and Seoul, because the missile's range puts much of Asia and the Pacific, including US military bases there, within reach.
Pyongyang earlier this year conducted a nuclear test, its fourth, and launched a long-range rocket that outsiders say was a cover for a test of banned missile technology.
A statement from South Korea's Joint Chiefs of Staff said a suspected first Musudan launch from the east coast city of Wonsan failed early Wednesday morning. It didn't elaborate. But Japan's Defense Ministry said the missile fragmented and pieces fell into waters off the Korean Peninsula's east coast.
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A US official also said the first launch appeared to be another failure, adding that the US was assessing exactly what had happened. The official wasn't authorized to comment publicly and requested anonymity. Another American official said the first launch was a suspected Musudan but initial indications were that it failed in flight over the Sea of Japan, which the Koreas call the East Sea.
In April, North Korea attempted unsuccessfully to launch three suspected Musudan missiles, but all exploded in midair or crashed, according to South Korean defense officials. Earlier this month, North Korea had another suspected Musudan failure, South Korean officials said.
Before April's launches, North Korea had never flight-tested a Musudan missile, although one was displayed during a military parade in 2010 in Pyongyang, its capital.