A North Korean propaganda website has warned of strikes against Southern islands and advised residents to leave.
It is the latest in a series of threats to come from North Korea and follows an announcement by the United States that it was boosting its missile defence system against Pyongyang.
The Uriminzokkiri website, linked to the regime, mentioned targets including Yeonpyeong island, which was attacked by Northern forces in 2010.
Pyongyang has made a series of threats since its last nuclear test in February prompted the United Nations to tighten sanctions, the BBC reports.
The United States said on Friday it would refocus missile defences to its west coast to counter the North's threats.
Defence Secretary Chuck Hagel said 14 more batteries would be placed in Alaska by 2017, adding to 30 already in place along the coast.
But the final phase of the US European Missile Defence programme is being scrapped to partly fund the project.