North Korea threatened "indiscriminate" nuclear strikes against South Korea and the US mainland if the two allies push ahead with joint military drills scheduled to begin Monday.The order for what it described as a "pre-emptive nuclear strike of justice" was made in a statement put out by the Supreme Command of the Korean People's Army.The threat came amid heightening tensions in the region as the secretive state fired six short-range missiles from its eastern port city of Wonsan last Thursday.The North launched the missiles just hours after the 15-member United Nations (UN) Security Council adopted its latest resolution aimed at curbing Pyongyang's nuclear weapon development.According to South Korea's defense ministry, the projectiles landed in the East Sea and no damage was reported.Earlier this year, North Korea ignored previous UN resolutions by carrying out its fourth ever nuclear test as well as a long-range rocket launch, drawing even tougher sanctions.Resolution 2270 will prevent the North from earning vital cash through a ban on exports of key resources such as coal and iron, while cargo shipments will also be screened.