The powerful sister of North Korean leader Kim Jong Un on Friday rejected Seoul's offer of economic assistance in return for denuclearisation steps, calling it the "height of absurdity".

The statement follows South Korean President Yoon Suk-yeol this week putting forward an "audacious" aid plan that would include food, energy and infrastructure help in return for the North abandoning its nuclear weapons programme.

Analysts previously said the chances of Pyongyang accepting such a deal – first floated during Yoon's inaugural speech – were vanishingly slim, as the North, which invests a vast chunk of its GDP into weapons programmes, has long made it clear it will not make that trade.

Kim Jong Un's sister, Yo Jong, on Friday called Yoon's offer the "height of absurdity", adding the entire premise that the North might willingly put its nuclear programme on the table was wrong.

"To think that the plan to barter 'economic cooperation' for our honour, [our] nukes, is the great dream, hope and plan of Yoon, we came to realise that he is really simple and still childish," she said in a statement carried by the official Korea Central News Agency.

"We make it clear that we will not sit face to face with him," she added, saying "no one barters its destiny for corn cake".

She further accused the South of recycling past proposals the North had already rejected, and compared Yoon to a barking dog.

South Korea's presidential office expressed "strong regret" over Yo Jong's "rude" remarks, but added that the offer of economic aid remained in place.

"North Korea's attitude is in no way helpful to the peace and prosperity of the Korean peninsula, as well as its own future, and only promotes isolation from the international community," it said in a statement.

Pyongyang last week warned it would "wipe out" Seoul authorities over a recent Covid-19 outbreak, a threat that came less than a month after Kim Jong Un said his country was "ready to mobilise" its nuclear capability in any war with the United States and the South. (AFP)