North Korea's chief negotiator has called the South Korean government "ignorant and incompetent", denounced US-South Korean air combat drills and threatened to halt all talks with the South unless its demands are met.
The comments by Ri Son-gwon, chairman of North Korea's Committee for the Peaceful Reunification of the country, were the latest in a string of inflammatory statements marking a drastic change in tone after months of easing tension with plans for denuclearisation and a summit scheduled with the US.
Mr Ri criticised the South for participating in the drills, as well as for allowing "human scum" to speak at its National Assembly, the North's KCNA news agency said in a statement.
"Unless the serious situation which led to the suspension of the north-south high-level talks is settled, it will never be easy to sit face to face again with the present regime of south Korea," the statement said.
It did not elaborate.
KCNA, in its English-language service, deliberately uses lower-case "north" and "south" to show that it only recognises one undivided Korea.
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North Korea on Wednesday said it might not attend the June 12 summit between leader Kim Jong-un and US President Donald Trump in Singapore if the US continued to demand it unilaterally abandon its nuclear arsenal.
A South Korean presidential Blue House official said the South intends to more actively perform "the role of a mediator" between the US and North Korea, but that goal has been cast into doubt by Mr Ri's comments.
"On this opportunity, the present south Korean authorities have been clearly proven to be an ignorant and incompetent group devoid of the elementary sense of the present situation," Mr Ri's statement said.
The statement did not identify the "human scum" by name, but Thae Yong-ho, a former North Korean diplomat to Britain who defected to the South in 2016, held a press conference on Monday at the South Korean National Assembly for his publication of his memoir.
In his memoir, Password from the Third Floor, Mr Thae describes North Korean leader Mr Kim as "impatient, impulsive and violent".
South Korean Foreign Minister Kang Kyung-wha told parliament that North Korea and the US had differences of views over how to achieve denuclearisation.
Mr Trump acknowledged on Wednesday it was unclear if the summit would go ahead.
Japan's Asahi newspaper reported on Thursday that the US had demanded North Korea ship some nuclear warheads, an intercontinental ballistic missile and other nuclear material overseas within six months, in return for the US removing it from a list of state sponsors of terrorism.
The North has long said it is open to eventually giving up its nuclear arsenal if the US withdraws its troops from South Korea and ends its "nuclear umbrella" alliance with Seoul.