The Kremlin says Russia is seriously worried about North Korea's readiness to use nuclear weapons after North Korean leader Kim Jong Un on March 4 ordered his military to be prepared to launch nuclear strikes at any time.

The threat by Kim Jong Un was his first official response to recently adopted United Nations sanctions against North Korea for its tests of nuclear weapons and long-range missiles.

North Korea also said on March 4 that it will continue to build up its nuclear weapons arsenal and take "strong and merciless physical" measures.

The remarks from the North Korean leader came on the same day that U.S. and South Korean officials began formal talks about deploying a sophisticated U.S. missile defense system in South Korea.

The U.S. and South Korea also plan to start large-scale military exercises next week that North Korea claims is preparation for an invasion.

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