A spokesman for North Korea's Committee for the Peaceful Reunification of Korea said for decades so-called reform-minded people in Seoul have wanted "to impose their corrupt system" on the North.
Such hopes are a "hallucination" by South Korea's "hostile forces," the spokesman was quoted as saying by the state-run Korean Central News Agency.
To expect policy change and reform in the North "is nothing but a foolish and silly dream, just like wanting the sun to rise in the west," he said.
Pyongyang has been reacting strongly to comments and speculation in South Korean and foreign media that change is slowly taking place north of the 1953 demarcation line that separates the two Koreas.
The newly appointed young leader of North Korea Kim Jong Un appears to be cultivating a more open and friendly public persona.
This is bolstered by the news confirmed by state media last week that he is married, ending speculation about the identity of the young woman accompanying him on tours and appearances around the country.
Kim Jong Un's Western presidential-style walkabouts are in stark contrast to his stern father, the late Kim Jong Il, whom he succeeded in December.
A picture carried by state media last week showed a smiling Kim Jong Un with his hat held in his hand waving to a crowd at the Rungna People's Pleasure Ground in Pyongyang.
His wife Ri Sol Ju is seen holding his arm as they walk.
Kim Jong Un's more frequent happy-couple appearances and recent removal of army chief Gen. Ri Yong Ho have fueled hopes in South Korea that North Korea's closed state-run economy is about to open up to the West, the BBC reported.
There also is increasing external pressure for change, not least from China, which remains North Korea's staunchest ally but also which has been pressing for reform in the secretive North for more than a decade, the BBC said.
Bloomberg reported that Gen. Hyon Yong Chol, a relatively unknown figure, was appointed vice marshal of the Korean People's Army only two days after the announced departure of Ri.
While the two positions are different in name, the timing indicates Hyon probably has replaced Ri, Koh Yu-hwan, professor of North Korean studies at Seoul's Dongguk University, told Bloomberg.
The move was the biggest public power shift since leader Kim Jong Un rose to power in December, Bloomberg said. It underscores his effort to cement control of the impoverished state he inherited from his father.
But the North Korean spokesman flatly denied change is on the way, calling such analysis "ridiculous rhetoric," the KCNA report said.
"Hostile forces such as the United States and the South Korean puppet group are running wild to isolate and stifle the Democratic People's Republic of Korea with vicious sanctions while preventing it from conducting normal exchange with other countries."