·South Korea and U.S. kicked off annual computer-simulated war game Monday.

·The exercise will mobilize 56,000 South Korean and 10,000 U.S. troops.

·DPRK said it put 1.2 million troops on a "special alert" in response to the exercise.

    SEOUL, Aug. 17 (Xinhua) -- An annual computer-simulated war game of South Korea and the United States was launched on Monday amid the Democratic People's Republic of Korea (DPRK)'s "nuclear deterrent" threats against it.

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    The Ulchi Freedom Guardian exercise, which will mobilize 56,000 South Korean and 10,000 U.S. troops, is to take place between Aug.17 and Aug. 27.

    The South Korean and U.S. military has been calling the annual drill as "purely defensive."

    Earlier in the day, through the nation's official media KCNA, the DPRK military said it put 1.2 million troops on a "special alert" in response to the exercise.

    The military also said "it would retaliate against any attempt to invade its territory with all offensive and defensive means including nuclear deterrent," according to South Korea's Yonhap News Agency.

    South Korean defense ministry spokesman Won Tae-jae, however, said that no unusual activity has been observed in the DPRK.

    "No unusual military activity has been spotted in the North (DPRK)," he said, adding that the "special alert" was launched whenever the joint war game was held in the past.

    The two Koreas remain at war after the 1950-53 Korean War ended in a truce, instead of a peace treaty.