"North Korea launched a projectile which was believed to be a submarine-launched ballistic missile (SLBM) around 6:30 pm (0930 GMT) in the East Sea (Sea of Japan)" near the northeastern port of Sinpo", a defence ministry spokesman said.
North Korea's fifth nuclear test seen imminent, increased movements at site
"We are keeping close tabs on the North Korean military and maintaining a full defence posture", he said.
It was not immediately known whether the launch was a success, he added.
North Korea has been pushing to acquire SLBM capability that would take its nuclear strike threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.
It has conducted a number of what it says were successful SLBM tests, but experts question the claim, suggesting Pyongyang had gone little further than a "pop-up" test from a submerged platform.
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