South Korea, which is still technically at war with the North, placed its troops and all government workers on emergency alert, Yonhap news agency reported. But Seoul's Defence Ministry said there were no signs of any unusual North Korean troop movements.
Japan's Foreign Minister Jun Azumi said his country had to be prepared for the unexpected on the security front.
Market players and regional powers will be on edge over what might happen next in the isolated state, whose collapsing economy and bid to become a nuclear weapons power pose major threats to northeast Asia.
Asian stocks and U.S. index futures fell, with South Korean shares tumbling as much as 5 percent, and the dollar gained after the announcement. The Korean won [KRW= 1174.50 16.00 (+1.38%)] fell 1.8 percent.
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