South Korea , South Korea 's and South Korea 's warn of `` significant consequences '' after a bomb test . U.S. officials say a new Stalinist nuclear test could come at any time . U.S. president tells his government to be prepared .
U.S. , South Korea and Japan warn of `` significant consequences '' after a bomb test . U.S. officials say a new North Korean nuclear test could come at any time . South Korea 's president tells his government to be prepared .