South Korea and the United States have begun annual military drills despite North Korea's threat to launch nuclear strikes in response to the exercises that it calls an invasion rehearsal.

Such fiery rhetoric by Pyongyang is not unusual. But the latest warning comes at a time of more tension following the defection of a senior North Korean diplomat and a U.S. plan to place a high-tech missile system in South Korea.

The North's military said in a statement Monday that it will turn Seoul and Washington into "a heap of ashes through a Korean-style pre-emptive nuclear strike" if they show any signs of aggression toward the North's territory.

The Ulchi Freedom Guardian drills that began Monday for a 12-day run is largely computer-simulated war games.