The missiles were fired off the country's east coast and flew about 70km (42 miles) into the Sea of Japan, a ministry spokesman said.

The first 10 rockets were fired at 6.20pm local time (0920 GMT), with eight more at 8.03pm and another seven at 9.30pm.

A series of recent rocket launches, including a ballistic missile that crossed the path of a Chinese passenger plane, have put South Korean troops on increased alert.

These have coincided with annual South Korean-US military exercises that started in February and will last until mid-April.

North Korea regularly condemns them as rehearsals for an invasion, while Seoul and Washington claim they are purely defensive.

Sunday's launch came two days after North Korea's National Defence Commission threatened to demonstrate the country's nuclear deterrent.

In a statement the commission said: "Additional measures will be taken to demonstrate its might one after another as long as the US nuclear threat and blackmail persist as now.

"The US had better roll back its worn-out hostile policy towards (North Korea) as soon as possible and shape a new realistic policy before it is too late."

North Korea carried out nuclear tests in 2006, 2009 and most recently in February 2013.

Six-party talks on the North's nuclear weapons programme have stalled, although there is a push from Pyongyang and its main ally, China, for these to resume.

The US and South Korea say North Korea must demonstrate a commitment to abandoning nuclear weapons before this can happen.