South Korea says the North has test-fired a short-range missile off its east coast, its fourth in two days.
A South Korean defense ministry spokesman said the missile was fired into the East Sea (Sea of Japan) on May 19.
A day earlier, Pyongyang fired three short-range missiles off the east coast.
South Korea's unification ministry spokesman, Kim Hyung-Seok, speaking before the latest launch, said the tests pose threats to the region and should be "stopped immediately."
U.S. and South Korean forces had been on heightened alert in recent weeks amid tensions triggered by North Korea's nuclear test in February.
UN Secretary-General Ban Ki-moon, speaking earlier in Moscow, said he was concerned by the North's latest actions and called on Pyongyang to return to talks with world powers.
Based on reporting by AFP and AP