SEOUL, South Korea — North Korea's military warned South Korea and the United States on Friday of "unprecedented nuclear strikes" as it expressed anger over a report the two countries plan to prepare for possible instability in the totalitarian country, a scenario it dismissed as a "pipe dream."The North routinely issues such warnings. Diplomats in South Korea and the US have repeatedly called on Pyongyang to return to international negotiations aimed at ending its nuclear programs."Those who seek to bring down the system in the (North), whether they play a main role or a passive role, will fall victim to the unprecedented nuclear strikes of the invincible army," North Korea's military said in comments carried by the official Korean Central News Agency.