SEOUL, South Korea — South Korea and the United States began annual military drills Monday despite North Korea's threat of 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 defense 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 North's "first-strike" units are ready to mount retaliatory attacks on South Korean and U.S. forces involved in the drills, according to the statement, carried by Pyongyang's state media.

South Korea's Unification Ministry expressed "strong" regret over the North's warning, saying the drills with the U.S. are defensive in nature. Seoul and Washington have repeatedly said they have no intentions of invading Pyongyang.

The 12-day Ulchi Freedom Guardian drills that began Monday are largely computer-simulated war games. The training involves 25,000 American troops and 50,000 South Korean soldiers, according to the U.S. and South Korean militaries.

The drills come just days after Seoul announced that Thae Yong Ho, No. 2 at the North's embassy in London, had recently defected to South Korea because he was disillusioned with the North's leadership. Pyongyang's state media called him "human scum" and a criminal who had been ordered home for a series of alleged criminal acts, including sexually assaulting a minor.

South Korea's President Park Geun-hye said Monday that there were signs of "serious cracks" in the North's ruling elite class after defections of key figures she didn't identity. Park told a security meeting that Pyongyang could carry out cyberattacks or other provocations on South Korea to divert public attention away from such domestic problems.

Many analysts said Thae's defection was an embarrassment to the North Korean government of leader Kim Jong Un, but would not weaken the unity of the country's elite class.

North Korea has already boosted its war rhetoric because of the planned deployment of the U.S. Terminal High-Altitude Area Defense system in South Korea, which Washington and Seoul says is needed because of the increasing North Korean threats.

About 28,500 U.S. troops are in South Korea to help deter potential aggression from North Korea, a legacy of the 1950-53 Korean War that ended with armistice, not a peace treaty.