South Korean defense officials say the North has test-fired at least 18 short-range missiles into the sea, the latest in a series of launches that have provoked criticism from Seoul and Washington.
South Korea's Yonhap news agency quoted the officials as saying the missiles were fired late Sunday from the eastern coast of the Korean peninsula and flew about 70 kilometers over the Sea of Japan.
North Korea is not banned from short-range missile launches under U.N. sanctions and frequently tests its arsenal.
South Korean troops have increased vigilance following a series of rocket or missile launches in late February and this month.
The activity coincides with annual South Korean-U.S. military exercises that started in February and will run until mid-April.
Pyongyang routinely condemns such war games as rehearsals for an invasion, while Seoul and Washington say they are purely defensive.