North Korea fired three short-range ship-to-ship missiles off its eastern coast into the Sea of Japan on Sunday, according to South Korea.

The North “fired three KN-01 missiles from its eastern border town of Wonsan between 4:21 p.m. and 4:47 p.m. today,” South Korea’s Joint Chiefs of Staff said in a statement.

Each of the missiles flew about 100 kilometers and landed in the sea, the report said.

The North carried out such short-range missile tests in February and in May, indicating that Sunday’s launches were possibly for testing further improvements.

The KN-01 is a modified version of the Chinese Silkworm surface-to-ship missile, which has a range of about 95 km. Full-scale development of the missile began in the late 1990s.