North Korea's chief negotiator called the South Korean government "ignorant and incompetent" on Thursday, denounced U.S.-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 denuclearization and a summit scheduled with the United States.

Ri criticized 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 recognizes one undivided Korea.

North Korea on Wednesday said it might not attend the June 12 summit between leader Kim Jong Un and U.S. President Donald Trump in Singapore if the United States continued to demand it unilaterally abandon its nuclear arsenal, which it has developed in defiance of U.N. Security Council resolutions to counter perceived U.S. hostility.

A South Korean presidential Blue House official said the South intends to more actively perform "the role of a mediator" between the United States and North Korea, but that goal has been cast into doubt by 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," 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," Thae describes North Korean leader Kim as "impatient, impulsive, and violent."