South Korea\'s Navy has agreed with the US to stage submarine drills twice a year beginning next year to strengthen readiness against possible North Korean provocations, Yonhap News Agency reported Friday.The announcement comes amid lingering tensions on the Korean Peninsula over the North\'s two deadly attacks on the South last year that killed a total of 50 people, mostly soldiers.North Korea has recently threatened to turn South Korea\'s presidential office into \"a sea of fire\" in anger over Seoul\'s massive military maneuvers last month near the tense western sea border. About 28,500 US troops are stationed in South Korea, a legacy of the 1950-53 Korean War that ended in an armistice, rather than a peace treaty.