Kim Jong-Nam, 45, was poisoned in Malaysia last month with VX, a nerve agent so deadly that it is classed as a weapon of mass destruction.
The dramatic killing at Kuala Lumpur airport prompted an international probe, lurid stories of North Korea's Cold War-style tradecraft and a bitter war of words between Malaysia and Pyongyang.
South Korean and Japanese media, citing diplomatic sources, have since reported that the US has been mulling placing the North back on its terror list, which includes Iran and Syria.
"The US will keenly realise how dearly it has to pay for its groundless accusations against the dignified" North if it puts it back on the terror list, the regime's foreign ministry spokesman told state-run newswire KCNA.
The spokesman maintained that Pyongyang opposed "all forms of terrorism" and accused the US of trying to tarnish its reputation.
South Korea has blamed the North for the murder, citing what they say was a standing order from leader Kim Jong-Un to kill his exiled half-brother who may have been seen as a potential rival.
However, the only North Korean arrested over the assassination on Saturday denounced Malaysia's probe into the murder as "a conspiracy to impair the dignity of the Republic (North Korea)".
Ri Jong-Chol, who was released and deported Friday due to lack of evidence, said that police had offered him a comfortable life in Malaysia in return for a false confession.
"But no way. No matter how good a life it could be, it is still not as good as my own motherland. How could I forget the motherland that raised me and fed me to this point?" he said to media in Beijing.
