North Korea warned Saturday the US will "pay dearly" if it puts Pyongyang on a terror list over the killing of its leader's half-brother, as a suspect in the murder claimed he was the victim of a conspiracy.

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.

Airline employee wanted

Ri's release came days after two women -- one Vietnamese and one Indonesian -- were charged with murdering Kim Jong-Nam.

Airport CCTV footage showed the women approaching the heavyset 45-year-old and apparently smearing his face with a cloth.

Police say he suffered a seizure and died less than 20 minutes later. Swabs of the dead man's face revealed traces of VX nerve agent.

On Friday police issued an arrest warrant for a North Korean airline employee, Kim Uk Il, 37, in connection with the murder.

They also requested that Hyon Kwang Song, second secretary at the North Korean embassy, assist the probe.

Both are believed to be in Malaysia. Four others are thought to have fled to Pyongyang on the day of the assassination.

North Korea, which has not acknowledged the dead man's identity, has vehemently protested the investigation, saying Malaysia is in cahoots with its enemies.

In response, Malaysia has cancelled a rare visa-free travel deal with North Korea -- a key conduit to the outside world -- and recalled its envoy to Pyongyang.

The nuclear-armed North was first designated a state sponsor of terrorism by the US in 1987 when its agents bombed a South Korean plane killing all 115 on board.

But it was taken off the list in 2008 after Pyongyang took steps toward freezing its nuclear facilities.

However, since then the North has resumed its activities, conducting four atomic tests and numerous missile tests despite the fact that they are banned under several UN Security Council resolutions.