Kim Yo-jong, North Korean leader Kim Jong-un's powerful sister, has slammed Seoul's offer of economic aid in return for denuclearization steps, calling it the "height of absurdity".
"No one barters their destiny for corn cake," she said in a statement.
South Korea's president Yoon Suk-yeol had Wednesday reiterated the plan, which he first mentioned in May.
But Ms Kim said on Friday that Yoon should "shut his mouth" and called him "simple and still childish".
South Korea's presidential office said it regretted her "rude" remarks, but that the offer still stands - the office was responding to Ms Kim's statement, released by state news agency KCNA on Friday.
"North Korea's attitude is in no way helpful to the peace and prosperity of the Korean peninsula, as well as its own future," it said.
In May, Yoon said South Korea was prepared to present what he called an "audacious plan" that would strengthen North Korea's economy and improve its people's quality of life.
Speaking at a news conference marking his first 100 days in office on Wednesday, the South Korean president reiterated that he was willing to provide phased economic aid to the North if it stopped developing nuclear weapons and took steps towards denuclearization.
Ms Kim's remarks on Friday were the first time a senior official from the North directly commented on Yoon's plan.
She said she did not know what else Mr Yoon would come up with if the plan did not work, but made clear that Pyongyang would "never deal with it".
The Korean peninsula has been divided between the north and the south since the end of World War II in 1945. The two sides remain technically at war after a conflict ended with an armistice in 1953.
While democratic South Korea's economy has flourished over the past few decades, the communist-ruled North has always struggled with food shortages. It has also consistently faced international sanctions over its nuclear program.
Last year, the UN's special rapporteur on human rights in the country, Tomas Ojea Quintana, said North Koreans were struggling to "live a life of dignity", with the sanctions and a Covid
-induced imports worsening food shortages.
Although Kim has admitted the country was dealing with shortages, he has refused to back down on nuclear weapons development.
Pyongyang has conducted regular missile tests over the years. Last month, Kim said the country was ready for military confrontation with the US, and prepared to mobilize its nuclear forces.
Ms Kim, born in 1987, is Kim's youngest sister and closest ally among his siblings. She has been touted among the top ranks of his potential successors.
Since 2014, her main role has been to protect her brother's image as a key official in the propaganda department. In this capacity, she has often delivered strongly-worded statements against the South and its allies, for example telling the US last year not to "cause a stink" by conducting joint military exercises with Seoul.