South Korean residents rushed to emergency shelters Thursday as part of an annual military exercise with the United States that has prompted dire warnings from rival North Korea.

Police and fire fighters directed residents to shelters throughout the capital, Seoul, while elsewhere U.S. troops joined more than half a million South Korean forces in the 10-day computer assisted exercise.

The United States and South Korea say the drill, held every year, is purely defensive in nature. But officials in North Korea denounced it this week as a “declaration of war” and said it will lead them to speed up their nuclear weapons program.

The rhetoric provokes a mixed reaction among South Korean civilians, who have lived for decades in a state of high tension with their northern neighbor. Some say the South should take care not to provoke the North too much, while others say it is necessary to continue the regular drills.

Pyongyang warned the military exercise would undermine efforts to restart talks aimed at dismantling North Korea's nuclear weapons programs.

Senior North and South Korean officials met last month in Indonesia for their first talks since North Korea shelled a border island killing four South Koreans in November. Relations already were strained over the sinking of a South Korean warship in March 2010.