The U.S. and South Korea began annual military drills today amid heightened inter-Korean tensions and threats of a nuclear strike from the North.

In a statement, U.S. and South Korean forces described the 12-day Ulchi Freedom Guardian war games as "non-provocative in nature" and designed to enhance "readiness, protect the region and maintain stability on the Korean peninsula."

Reporting from Seoul, NPR's Elise Hu tells our Newscast unit that tens of thousands of U.S. and South Korean personnel participate in the training:

"[The games] officially got under way on Monday. North Korea's warning from its state media outlet KCNA says that the U.S. and South Korea should bear in mind that if they show the slightest sign of aggression, North Korea would turn the South into a 'heap of ashes through a Korean-style pre-emptive nuclear strike.' "

It's not the first time that Pyongyang has threatened nuclear retaliation over U.S.-South Korean drills. As The Two-Way has reported, North Korea warned of a "pre-emptive nuclear strike of justice" during military exercises in March.

Elise says the latest war games come amid high tensions on the peninsula.

"A new round of sanctions on North Korea went into effect this spring," she notes. "And last week, South Korean officials announced North Korea's deputy ambassador to the United Kingdom has defected to South Korea."

In addition to Thae Yong Ho, a number of North Koreans reportedly have defected this year — including a high-ranking military officer in April and 13 North Korean workers at a state-run restaurant in China.

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