North Korea says it will not resume talks with the South until issues between the two countries are resolved.
Its chief negotiator dismissed the South Korean authorities as incompetent and senseless. Pyongyang is angry at continuing US-South Korea joint military exercises.
Planned talks between President Donald Trump and North Korean leader Kim Jong-un have also been thrown into doubt.
South Korea has offered to act as a bridge between the two sides.
But that now looks less likely following North Korea’s statement.
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Chief negotiator Ri Son-gwon reverted to the angry language the north has used before, in comments reported by state news agency KCNA.
He also criticised the South Korean authorities for allowing “human scum” (a reference to a North Korean defector) to speak at the Seoul National Assembly.
On Tuesday North Korea pulled out of a meeting with South Korea scheduled for the following day because of anger over the start of US-South Korea joint military drills.
It later released a statement saying it might pull out of the summit with the US if the Trump administration insisted it gave up its nuclear weapons unilaterally.
The highly anticipated meeting between Mr Trump and Mr Kim is due to take place in Singapore on 12 June.
Thursday’s criticism of South Korea marks yet another shift in tone from Pyongyang just weeks before the planned US summit, the BBC’s Laura Bicker in Seoul says.
Pyongyang appears to be putting pressure on both the US and South Korea and refusing to talk until some concessions are made, she adds.
What has upset North Korea?
The current US-South Korean aerial combat drills known as Max Thunder involve nearly 100 warplanes carrying out exercises over the Korean peninsula.
North Korea described them as a “provocation” and a rehearsal for an invasion.
North Korea’s suggestion that it might pull out of the US summit pointed the finger squarely at US National Security Adviser John Bolton.
“We do not hide our feeling of repugnance towards him,” said Wednesday’s statement, which was written by Vice-Foreign Minister Kim Kye-gwan.
North Korea was apparently alarmed by an interview Mr Bolton gave comparing it to “the Libya model” of denuclearisation.
Libya’s leader Muammar Gaddafi gave up his nuclear programme only for him to be killed by Western-backed rebels a few years later.
What does this mean for the nuclear issue?
At the summit between the leaders of North and South Korea on 27 April both sides agreed to work to rid the Korean peninsula of nuclear weapons.
But Pyongyang’s professed commitment to “denuclearisation” is likely to differ from Washington’s demand for “comprehensive, verifiable and irreversible” nuclear disarmament.
South Korea’s Foreign Minister Kang Kyung-wha told MPs on Thursday there was a gap between North Korea and the US on “how to pull off denuclearisation”, according to South Korea’s Yonhap news agency.
North Korea said last week it would begin dismantling its nuclear test site at Punggye-ri between 23-25 May, but there was no mention of allowing foreign experts access to the site.
On a recent visit to North Korea, US Secretary of State Mike Pompeo stressed that any claim of denuclearisation by the North would require a “robust verification” programme by the US and other nations.
North Korea turns on ‘incompetent’ South}