North Korea, facing international censure for this week's nuclear test, threatened on Wednesday to attack the South after it joined a U.S.-led plan to check vessels suspected of carrying equipment for weapons of mass destruction.
Adding to tension in the region, South Korean media reported that Pyongyang had restarted a plant that makes plutonium that can be used in nuclear bombs.
In Moscow, news agencies quoted an official as saying that Russia is taking precautionary security measures because it fears mounting tensions over the test could escalate to war.