Pyongyang "fires unidentified ballistic missile into (the) East Sea," Seoul's Joint Chiefs of Staff said, referring to the body of water also known as the Sea of Japan, without giving further details.

The launch came about a week after North Korean leader Kim Jong Un ordered navy vessels to be speedily armed with nuclear weapons, as the country conducted its first test-firing of a new warship weapons system.

Last month, Pyongyang unveiled a 5,000-ton destroyer-class vessel named Choe Hyon that some analysts said could be equipped with short-range tactical nuclear missiles.

The two Koreas are technically still at war as the 1950-1953 conflict ended in an armistice rather than a peace treaty.

With relations souring, the North last year blew up roads and railways connecting it to the South and fortified areas of the border with more mines.

In early April, South Korea's military said its troops fired warning shots when around 10 North Korean soldiers briefly crossed the heavily fortified border that divides the peninsula.