North Korea has threatened to use its nuclear deterrent in response to war games by the US and South Korea. The US and South Korea are beginning a major military exercise in the Sea of Japan, despite threats of retaliation from North Korea. The navy and air force manoeuvres involve 20 ships, 200 planes and 8,000 US and South Korean personnel. Washington and Seoul say they want to send a clear signal to the North Korea following the sinking of a South Korean warship in March. An international investigation said a North Korean torpedo sank the ship. However, the claim has been angrily denied by Pyongyang. On Saturday, North Korea threatened to use its nuclear deterrent in retaliation in response to the exercise. Officials said, the four-day drill includes the aircraft carrier USS George Washington and some 200 fixed-wing aircraft. The sinking of the Cheonan warship claimed the lives of 46 South Korean sailors.