South Korean and US troops launched their annual joint drills on Friday, the Seoul-based Yonhap News Agency reported, amid heightened tension following North Korea\'s third nuclear test last month.  The allies started the Foal Eagle drills, which are field training exercises involving tens of thousands of troops from their ground, naval and air forces, a Seoul military official was quoted as saying. They will last until April 30.  The Key Resolve/Foal Eagle exercises come after North Korea warned the top US commander in South Korea of \"miserable destruction\" if the American military goes ahead with the two-month-long joint drills.  Separately, South Korean and US troops will conduct computer-simulated drills named Key Resolve from March 11-21. About 10,000 US troops and 200,000 South Korean troops joined the Foal Eagle, according to the official.  South Korean and US marines are scheduled to stage a large-scale landing drill and military logistics trainings in case of an emergency on the Korean Peninsula. The Key Resolve drill will involve about 3,500 US troops and 10,000 South Korean troops.  About 28,500 US troops are stationed in South Korea, a legacy of the 1950-53 Korean War. This week, North Korea ratcheted up its harsh rhetoric against the drills, saying, \"The black cloud of a nuclear war is coming,\" and warning any provocations against the North will lead to the self-destruction of the aggressor.  The North\'s threats came as South Korea and the US are pushing for tougher UN sanctions to punish North Korea for conducting its third nuclear test on Feb. 12.