North Korea today test-fired ten rockets into the sea, South Korea's defence ministry said, the latest in a series of launches that have provoked criticism from Seoul and Washington.

The rockets were fired off the North's east coast and flew about 70 kilometres into the Sea of Japan (East Sea), a ministry spokesman said.

"Following the North's rocket launch at 6:20 pm, our military has maintained a close watch for possible North Korean provocations," he said. Yonhap news agency earlier reported that 10 short-range missiles were fired.

South Korean troops have increased vigilance following a series of launches of rockets or missiles. The activity coincides with annual South Korean-US military exercises that started in February and will run until mid-April.

Pyongyang routinely condemns such joint exercises as rehearsals for an invasion, while Seoul and Washington say they are purely defensive.