The pariah state was sanctioned by the US and the United Nations on May 9 after it released a series of photographs claiming it had conducted an underwater test-fire of a submarine-launched ballistic missile.
However the claims were widely questioned by experts and never verified, and now, the US is says it has proof that the images were `manipulated`.
US Admiral James Winnefeld told an audience at the Centre for Strategic & International Studies in Washington on Tuesday that North Korea was still "many years" from developing submarine-launched ballistic missiles.
"They have not gotten as far as their clever video editors and spinmeisters would have us believe," said Winnefeld, who is vice chairman of the Joint Chiefs of Staff.
Analysis seen from German aerospace engineers Markus Schiller and Robert Schmucker of Schmucker Technologie appeared to support Winnefeld`s statement.
The Munich-based pair said photos of the launch were "strongly modified", including reflections of the missile exhaust flame in the water which did not line up with the missile itself.
North Korea, which regularly threatens to destroy the United States, had a record of offering faked proof to claim advances in missile technology, such as poorly built mockups of missiles on display at military parades in 2012 and 2013.
The pair agreed with analysis posted by experts on the websites 38north.org and armscontrolwonk.com that the missile was likely launched from a specially designed submerged barge, and not from a submarine
A photo on state television showed a missile high in the sky leaving a trail of white smoke, whereas other photos from state media showed no white smoke, suggesting the two photos were of different missiles with different propulsion systems, Schiller and Schmucker said.
South Korea stood by its position that the photos appeared authentic. "We haven`t changed our stance that the rocket was fired from a submarine and flew about 150 meters out of the water,” a South Korean military official said.
The North`s National Defence Commission, the main ruling body headed by leader Kim Jong Un, said on Wednesday the submarine-based missile launch was "yet a higher level of accomplishment in the development of strategic attack means".
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