North Korea has threatened to pull out of the armistice agreement that ended the Korean War, accusing the United States of breaking its terms. A statement, carried by the official Korean Central News Agency (KCNA), accused the United States of bolstering its forces in the region and mounting a naval blockade. Earlier on Monday, the United States and South Korea announced they would stage joint military exercises in South Korea next month, in a move which was thought likely to heighten tensions with the North. Tensions on the peninsula have risen dramatically since North Korea revived its nuclear programme and pulled out of the nuclear non-proliferation treaty (NPT) late last year. The Korean War ended with an armistice in 1953 - not a peace treaty.