The United States and South Korea have decided to stage joint military exercises in South Korea next month, in a move which is likely to heighten tensions with the North. US officials insisted that the exercises were a planned, annual event, unrelated to a stand-off over North Korea's nuclear ambitions. But they are likely to infuriate the North, which has already warned the US against any military build-up in the region. The US is also reported to have agreed with its other regional ally, Japan, to conduct joint tests designed to intercept ballistic missiles, a Japanese newspaper said on Monday.