North Korea fires 23 missiles, prompting air raid alert in South Air raid sirens sounded on a South Korean island and residents evacuated to underground shelters after North Korea fired more than 20 missiles Wednesday, at least one of them in its direction and landing near the rivals' tense sea border.The launches came hours after North Korea threatened to use nuclear weapons to get the U.S. and South Korea to "pay the most horrible price in history" in protest of ongoing South Korean-U.S. military drills that it views as an invasion rehearsal.Later Wednesday, South Korean fighter jets launched three air-to-surface, precision-guided missiles near the eastern sea border to show its determination to get tough on North Korean provocations.Analyst Cheong Seong-Chang at the private Sejong Institute in South Korea said the danger of armed clashes between the Koreas off their western or eastern coasts is increasing.Animosities on the Korean Peninsula have been running high in recent months, with North Korea testing a string of nuclear-capable missiles and adopting a law authorizing the preemptive use of its nuclear weapons in a broad range of situations."