Malaysia has asked Interpol to put an alert out for four North Korean suspects over the murder of Kim Jong-nam, Malaysia's police chief said.

The estranged half-brother of North Korean leader Kim Jong-un was killed in Kuala Lumpur's main airport last week, in what South Korean and US officials say was an assassination carried out by North Korean agents.

Police chief Khalid Abu Bakar said on Thursday that police have sent an official request to the North Korean embassy in Malaysia to question the embassy's second secretary and an official from North Korea's national airline.

His statements came as North Korea denied involvement, saying Malaysia's investigation into the death of one of its nationals is full of "holes and contradictions".

Malaysian police have not directly pinpointed North Korea as being behind the death, but have already arrested a North Korean man working at a Malaysian company along with three other Southeast Asian people. They are searching for several more North Koreans.

READ MORE: Kim Jong-nam's killing one week on - What we know

The Korean Jurists Committee, a legal body affiliated with North Korea's rubber-stamp parliament, said in a statement that the Malaysian investigation lacks fairness and was influenced by the South Korean government, which blames Pyongyang for the death.

The North has not acknowledged that the dead man is Kim. Thursday's statement described the man only as a North Korean citizen bearing a diplomatic passport.

It said that South Korea had "kicked up a fuss" and had plotted to have North Korea blamed for the killing.

North blames Malaysia

"The biggest responsibility for his death rests with the government of Malaysia as the citizen of the DPRK died in its land," the statement said.

The DPRK refers to the Democratic People's Republic of North Korea, the country's official name.

Malaysian police said on Wednesday that the two women suspected of fatally poisoning Kim were trained to coat their hands with toxic chemicals and then wipe them on his face.

Police say the substance used remains unknown, but it was potent enough to kill Kim before he could make it to a hospital.

Khalid told reporters that the women, one Vietnamese and the other Indonesian, knew they were handling poisonous materials and "were warned to take precautions."

The women and a Malaysian man, believed to be the boyfriend of the Indonesian woman, have been arrested.

Surveillance video showed both women keeping their hands away from their bodies after the attack, he said, then going to restrooms to wash.

Those details are not clear in video obtained by media outlets.

Kim Jong-nam killing: Two North Koreans sought by police

But the North Korean embassy in Kuala Lumpur has already ridiculed the police account and demanded the immediate release of the two "innocent women".

An embassy statement asked how the women were able to survive if they also had the deadly toxins on their hands.

Malaysian police said the women washed their hands soon after poisoning Kim.

'A planned thing'

Khalid said the women had practised the attack at two Kuala Lumpur malls. "We strongly believe it is a planned thing and that they have been trained," he said.

Khalid couldn't confirm whether North Korea's government was behind Kim's death but added, "What is clear is that those involved are North Koreans."

At least one of the women has said that she was tricked into attacking Kim, believing she was taking part in a comedy prank TV show.

The case has perplexed toxicologists, who question how the two women could have walked away unscathed after handling a powerful poison.

Kim had spent most of the past 15 years living in China and Southeast Asia. He is believed to have had at least three children with two women. No family members have come forward to claim the body.

WATCH: Are sanctions against North Korea working?

Analysts in South Korea's Seoul said Kim Jong-un probably had his brother killed because he could be a potential challenger to his rule in a country where his family ruled for three generations and where the bloodline is still extolled in the country's founding mythology.

North Korea has a long history of ordering killings of people it views as threats to its regime.

While Kim was not thought to be seeking influence, his position as eldest son of the family that has ruled North Korea since its founding could have made him appear to be a danger.

He was at the airport to fly to Macau, where he had a home.

Source: AP news agency