North Korea has demanded the removal of US Secretary of State Mike Pompeo from nuclear talks, accusing him of "talking nonsense" and being "reckless".
A senior foreign ministry official said that further talks would be "lousy" if Mr Pompeo was involved, asking that he be replaced by someone "more careful".
It comes after North Korea said it had tested a new "tactical guided weapon".
The test is the first since a February summit in Hanoi between the countries' leaders ended without an agreement.
Mr Pompeo visited North Korea four times last year - including for a meeting with North Korean leader Kim Jong-un.
Why is Pompeo so controversial?
Last week, during a US Senate subcommittee hearing, Mr Pompeo was asked if he would agree with descriptions of Mr Kim as a "tyrant", he then responded: "Sure, I'm sure I've said that."
This was met with a strong reaction from North Korean foreign ministry official Kwon Jong-gun, who said that Mr Pompeo "spouted reckless remarks, hurting the dignity of our supreme leadership... to unveil his mean character".
Mr Kwon also blamed Mr Pompeo for the abrupt ending to the Hanoi summit.
What to make of the Hanoi summit collapse?
On Thursday, he said that if Mr Pompeo were to be involved in further talks, "the table will be lousy once again and the talks will become entangled".
"Even in the case of possible resumption of dialogue with the US, I wish our dialogue counterpart would not be Pompeo but ... [another] person who is more careful and mature in communicating with us," Mr Kwon said, according to the Korean Central News Agency (KCNA).
During discussions in Pyongyang in July, Mr Pompeo was condemned for his "gangster-like" insistence that the country move towards denuclearisation.
Testing a 'new weapon'
State media has given few details on Thursday's reported test of a new "tactical guided weapon", but analysts say it is unlikely to be a return to the long-range missile tests seen as a threat to the US.
According to KCNA, the test was overseen by Mr Kim himself. It said the test was "conducted in various modes of firing at different targets", which analysts believe means the weapon could be launched from land, sea or air.
It is unclear if the weapon was a missile, but most observers agree that it was probably a short-range weapon.
Last year, Mr Kim said he would stop nuclear testing and would no longer launch intercontinental ballistic missiles.
Nuclear activity appears to be continuing, however, and satellite images at North Korea's main nuclear site last week showed movement, suggesting the country could be reprocessing radioactive material into bomb fuel.
The country claims it has developed a nuclear bomb small enough to fit on a long-range missile, as well as ballistic missiles that could potentially reach the mainland US.
Where do the nuclear talks stand?
Very little has moved since the US and North Korean leaders walked out of talks in Hanoi - apart from rhetoric.
The US said talks broke down because North Korea wanted all sanctions lifted in exchange for scaling back its nuclear capacity, but Pyongyang disputes that.
In his most recent comments, Mr Kim urged Mr Trump to pursue a deal that was "mutually acceptable" but also spoke of his excellent ties with the US leader. Mr Trump responded by tweeting generous praise of Mr Kim and also welcomed the idea of another summit.
Earlier this week the State Department announced that Stephen Biegun, its special envoy for North Korea, was heading to Moscow for meetings "with Russian officials to discuss efforts to advance the final, fully verified" denuclearisation of North Korea.
It came as speculation of a possible future meeting between Kim Jong-un and Vladimir Putin intensified.