Military tensions have been soaring on the divided Korean peninsula since the North carried out its fourth nuclear test on January 6, followed a month later by a long-range rocket launch that was widely seen as a disguised ballistic missile test.

US defence officials said they had tracked two launches – both believed to be medium-range Rodong missiles fired from road-mobile launch vehicles.

The Rodong is a scaled-up Scud variant with a maximum range of around 1,300 kilometres.

South Korean military officials said the first missile was launched from Sukchon in the country's southwest at 5:55 am (local time) and flew 800 kilometres before splashing down into the East Sea (Sea of Japan).

The second, fired about 20 minutes later, disappeared off radar early into its flight, the officials said.

Today's launches came a day after US President Barack Obama signed an order implementing tough sanctions adopted earlier this month against North Korea by the UN Security Council.

To register its anger at the joint exercises, the North fired two short-range missiles into the East Sea on March 10.

A few days later, Kim announced that a nuclear warhead explosion test and firings of "several kinds" of ballistic missiles would be carried out "in a short time".

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