Tensions run high in Asia as South Korea, US begin annual military drills

North Korea threatened Monday to launch a nuclear first-strike and turn Seoul and Washington, D.C., into "a heap of ashes" — a threat that comes on the heels of China using its military to signal that it, too, would go to war to enforce its territorial claims.

While Pyongyang’s threat isn't new, it comes as South Korea and the U.S. begin joint military exercises amid the defection of a senior North Korean diplomat and a U.S. plan to place a defense missile system in South Korea.

Tension in the region already was heightened after China last week flashed its newest offensive weapons during war games in the Sea of Japan, exercises that put on display Beijing's latest-generation frigate and long-range bombers as a warning to its neighbors —and the U.S. — that if is not ready to negotiate its territorial claims.

China has grown increasingly assertive over its claim to a chain of uninhabited islands controlled by Japan, recently dispatching more than a dozen coast guard vessels as Chinese fishing vessels swarmed the surrounding waters.

South Korea and the U.S., meanwhile, began their joint exercise despite Pyongyang’s fiery rhetoric, which claimed the North's "first-strike" units were ready to mount retaliatory attacks on South Korean and U.S. forces involved in the drills.

South Korea expressed “strong” regret over Pyongyang’s warning, saying the drills with the U.S. are purely defensive in nature. Seoul and Washington have repeatedly said they have no intentions of invading Pyongyang.

This year’s Ulchi Freedom Guardian drills that began Monday for a 12-day run are largely computer-simulated war games. The training involves 25,000 American troops and 50,000 South Korea soldiers, according to the U.S. and South Korean militaries. It comes 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.

The Associated Press contributed to this report.