North Korean leader Kim Jong-Un ordered preparations on Friday for strategic rocket strikes on the US mainland and military bases after US stealth bombers flew training runs over South Korea.

North Korean leader Kim Jong-Un ordered preparations on Friday for strategic rocket strikes on the US mainland and military bases after US stealth bombers flew training runs over South Korea.

Kim directed his rocket units on standby at an overnight emergency meeting with top army commanders, hours after nuclear-capable US B-2 stealth bombers were deployed in ongoing US joint military drills with South Korea.

In the event of any "reckless" US provocation, North Korean forces should "mercilessly strike the US mainland... military bases in the Pacific, including Hawaii and Guam, and those in South Korea", the North Korean leader was quoted as saying by the official Korean Central News Agency (KCNA).

Kim argued that the stealth bomber flights went beyond a simple demonstration of force and amounted to a US "ultimatum that they will ignite a nuclear war at any cost".

Meanwhile, an unidentified South Korean military official quoted by Yonhap news agency said a "sharp increase" in personnel and vehicle movement had been detected at the North's mid- and long-range missile sites.

Earlier on Thursday, the US said two nuclear-capable B-2 stealth bombers flew "deterrence" missions over South Korea.