SEOUL/WASHINGTON - North Korea put its missile units on standby on Friday to attack US military bases in South Korea and the Pacific, after the United States flew two nuclear-capable stealth bombers over the Korean peninsula in a rare show of force.North Korean leader Kim Jong-un signed off on the order at a midnight meeting of top generals and "judged the time has come to settle accounts with the US imperialists in view of the prevailing situation", the official KCNA news agency said.KCNA said North Korea and the United States could only settle their differences by "physical means". The North has an arsenal of Soviet-era short-range Scud missiles that can hit South Korea but its longer-range Nodong and Musudan missiles, which could in theory hit US Pacific bases, are untested.China, the North's sole major ally, repeated its calls for restraint on the Korean peninsula at a regular foreign ministry briefing on Friday and made no criticism of the US flights."We hope that relevant parties will work together in pushing for a turnaround of the tense situation," foreign ministry spokesman Hong Lei told reporters.On Thursday, the United States flew two radar-evading B-2 Spirit bombers on practice runs over South Korea, responding to a series of North Korean threats. They flew from the United States and back in what appeared to be the first exercise of its kind, designed to show America's ability to conduct long-range, precision strikes "quickly and at will", the US military said.The news of Kim's response was unusually swift."He finally signed the plan on technical preparations of strategic rockets of the KPA (Korean People's Army), ordering them to be on standby for fire so that they may strike any time the US mainland, its military bases in the operational theaters in the Pacific, including Hawaii and Guam, and those in South Korea," KCNA said.South Korea's Yonhap news agency reported there had been additional troop and vehicle movements at the North's mid- and long-range missile sites, indicating they may be ready to fire.It was impossible to verify the report which did not specify a time frame, although South Korea's Defense Ministry said on Friday that it was watching shorter-range Scud missile sites closely as well as Nodong and Musudan missile batteries.The North has launched a daily barrage of threats since early this month when the United States and the South, allies in the 1950-53 Korean War, began routine military drills.The South and the United States have said the drills are purely defensive in nature and that no incident has taken place in the decades they have been conducted in various forms.The United States also flew B-52 bombers over South Korea earlier this week.The North has put its military on highest readiness to fight what it says are hostile forces conducting war drills. Its young leader has previously given "final orders" for its military to wage revolutionary war with the South.North Korea conducted a third nuclear weapons test in February in breach of U.N. sanctions and despite warnings from China, its one major diplomatic ally.