Reconnaissance satellites have identified the huge missile at a government research and development facility in Pyongyang, South Korean government sources told the Chosun Ilbo newspaper.
Analysts estimate the weapon to be around 130 feet long and, equipped with a more powerful booster unit, capable of delivering a warhead more than 6,200 miles.
Unveiling the missile will raise new fears in neighbouring countries, already alarmed by Pyongyang's insistence that it will go ahead with the launch in mid-April of a rocket to put a satellite into orbit.
Washington, Seoul and Tokyo believe the launch is a covert test launch of a missile.
Analysis of the rocket, which is being prepared for launch at the new Tongchang-ri aerospace facility in the far north-west of the country, indicates that at 105 feet long it is identical in length to the Taepodong-2 missile that was fired in April 2009 and, with a range of 4,163 miles, is the most potent missile in the North Korean armoury at present.