Jeffrey Lewis, US top expert on Kim Jong-un's military and missiles, has claimed that North Korea is just one step away from launching a nuclear-tipped intercontinental ballistic missile (ICBM) that could reach the United States mainland.

Mr. Lewis believes that the last remaining obstacle is a warhead that is capable of handling intense heat during re-entry into the atmosphere. North Korea has compact fission warhead which is capable of fitting on future ICBM. One of the major questions now, Mr. Lewis said, if the warhead could be strong enough to withstand the vibration, shock, and extreme heat temperatures that a nuclear warhead would encounter during intercontinental trajectory.

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North Korea has been rapidly testing different nuclear missiles program at an unprecedented speed for the past moths. Four new weapon systems have also been tested over the past few weeks like the super-accurate short-range ballistic missiles. Its latest intermediate-range ballistic missile is believed that it could hit US bases in Guam and NK has successfully tested new anti-ship cruise missiles.

The US expert also issued a warning that North Korea could soon be "knocking on the door" of the United States with new super-powered ICBMs, though some say that Kim's missile program is overrated. Furthermore, similar doubts were shown on nuclear warheads of China in 1960, urging the leader of China to arm a missile alongside live nuclear weapon and launch it across the country to dismiss lingering doubts. But, North Korea has not taken a similar step.

Meanwhile, the news arrives after Kim Jong-un together with his closest military officials were seen while enjoying the nation's annual air combat contest, such as war games and destructive combat training, creating explosions an airfield.