“It would have been more favourable for his image to shut his mouth,” Kim Yo Jong said in a statement released by state news agency KCNA, calling Yoon “really simple and still childish” to think that he could trade economic cooperation for the North’s honour and nuclear weapons.

“No one barters its destiny for corn cake,” she added.

South Korea’s Unification Minister, who handles relations with the North, called Kim’s comments “very disrespectful and indecent.”

While Yoon has said he is willing to provide phased economic aid to North Korea if it ended nuclear weapons development and began denuclearisation, he has also pushed to increase South Korea’s military deterrence against North Korea. South Korea has has resumed long-suspended joint drills with the United States, including major field exercises due to begin next week.

On Wednesday a US State Department spokesman said Washington supports Yoon’s policies, but Kim said the joint drills show that the allies’ talk of diplomacy is insincere.

“We make it clear that we will not sit face to face with him,” she said of Yoon.

Experts say South’s latest economic plan is similar to proposals by previous leaders, including those during the summits between the then-US president Donald Trump and North Korean leader Kim Jong Un, suggesting the North was unlikely to accept the offer.

“Yoon’s initiative adds to a long list of failed offers involving South Korean promises to provide economic benefits to North Korea ... These were the same assumptions that were behind a succession of failed efforts to jump-start denuclearisation talks,” Scott Snyder, a senior fellow at the Council on Foreign Relations think tank, said in a blog post on Thursday.

North Korea test fired two cruise missiles into the sea on Wednesday, the first such test in two months. It came after the country declared victory over Covid-19 last week.