North Korea threatened Wednesday to block cross-border traffic and blow up any South Korean loudspeakers blasting propaganda northward after a six-year hiatus, as tensions soared over the sinking of a South Korean warship. The dramatic deterioration in relations came as U.S. Secretary of State Hillary Rodham Clinton visited Seoul at the end of a three-nation Asian tour that was dominated by the March 26 sinking blamed on a North Korean torpedo attack. "This was an unacceptable provocation by North Korea, and the international community has a responsibility and a duty to respond," Clinton told reporters after talks with South Korean leaders.