NORTH Korean leader Kim Jong-Un has ordered preparations for strategic rocket strikes on the US mainland and military bases after US stealth bombers flew training runs over South Korea. The order on Friday came as US Defense Secretary Chuck Hagel, with tensions soaring on the Korean peninsula, said Washington would not be cowed by Pyongyang's bellicose threats and stood ready to respond to "any eventuality." 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," he was quoted as saying by the official Korean Central News Agency (KCNA). While North Korea has no proven ability to conduct such strikes, Kim said: "The time has come to settle accounts with the US imperialists." The youthful leader 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." 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. The United States rarely acknowledges B-2 flights to the Korean peninsula, which remains technically at war. The flights came as part of annual drills between the United States and South Korea, which North Korea each year denounces as rehearsals for war. Pyongyang has been particularly vocal this time, angered by UN sanctions imposed after its long-range rocket launch in December and the third nuclear test it carried out last month. The bulk of the threats emanating from Pyongyang have been dismissed as bluster. But Washington has opted to match the threats with its own muscle-flexing. "We will be prepared - we have to be prepared - to deal with any eventuality," Hagel told reporters at the Pentagon. "We must make clear that these provocations by the North are taken by us very seriously and we'll respond to that," Hagel said, defending the B-2 deployment.