US intelligence officials have been looking into such rumours for more than a week, the official told CNN.
"With that society, you can never be 100 percent sure, but we just don't see any evidence of it. It's a closed society, but at this point we do not believe it's true," he said.The official said preliminary analysis of the rumour suggested it was part of a "calculated effort to disrupt the economy of South Korea at a fragile time by suggesting things are going haywire up north".
Further, there was no evidence of movement of North Korean forces "that you would associate with the turmoil of a calculated assassination", he said.
Kim, believed to be in his late 20s, assumed the title of "supreme leader" after his father Kim Jong Il's death in December at the age of 69. Kim Jong Il ruled the country since 1994.