A government body dismissed reports that the present leadership was breaking with the past as “ridiculous”.
Some commentators have speculated that the recent removal of North Korea’s top general pointed to a possible power struggle over economic reform.
Kim Jong-un succeeded his late father, Kim Jong-il, in December.
Mr Kim, believed to be in his late 20s, has since adopted a warm public persona, being photographed at fun fairs and pop concerts with his young wife.
That – together with the recent removal of army chief General Ri Yong-ho – has fuelled hopes in the South that he could be planning to open up North Korea’s closed state-run economy.
However, a spokesman for North Korea’s Committee for the Peaceful Reunification of Korea, which deals with cross-border affairs, dismissed the speculation in an interview with state-run KCNA news agency on Sunday.
He said that Kim Jong-un would pursue the “military first” policy brought in by his father and would build a “civilised and comfortable life for the people under socialism”.
‘Silly dream’
“The puppet group (South Korea)… tried to give (the) impression that the present leadership of the DPRK (North Korea) broke with the past. This is the height of ignorance,” the spokesman said.
“To expect policy change and reform and opening from the DPRK is nothing but a foolish and silly dream, just like wanting the sun to rise in the west.”
He accused Seoul of trying to impose capitalism on Pyongyang by “trumpeting reform and opening”, adding: “There cannot be any slightest change in all policies.”
BBC Asia analyst Charles Scanlon says the statement probably shouldn’t be taken at face value.
Any reference to reform has always been anathema for North Korean officials, he says, and it is a word they associate with victory for their capitalist enemies in the South.
There is also substantial external pressure for change, not least from China, which appears close to Kim Jong-un and his inner circle and which has pressed for reform in North Korea for more than a decade, our correspondent adds.
Mr Kim and those around him are being keenly watched for the direction in which they will take the communist state