Four North Korean suspects in the murder of the estranged half-brother of North Korean leader Kim Jong-un fled Malaysia on the day he was attacked at Kuala Lumpur airport and apparently killed by a fast-acting poison, police say.

A North Korean man, a Vietnamese woman and an Indonesian woman have been arrested already in connection with the death of Kim Jong-nam last Monday, which has triggered a diplomatic spat between Malaysia and Pyongyang.

South Korean and US officials believe Kim Jong-nam was killed by agents from the reclusive North, whose diplomats in Kuala Lumpur have sought to prevent an autopsy on the 46-year-old's body and demanded it be handed over.

"We believe the North Korean regime is behind this incident, considering five suspects are North Koreans," Jeong Joon-hee, spokesman at South Korea's Unification Ministry that handles inter-Korea affairs, told a briefing on Sunday.

Three arrests amid probe into death of North Korean leader's half-brother

Kim Jong-nam, the eldest son of the late North Korean leader Kim Jong-il, had spoken out publicly against his family's dynastic control of the isolated, nuclear-armed nation.

The young, unpredictable North Korean leader had issued a "standing order" for his elder half-brother's assassination, and there was a failed attempt in 2012, according to some South Korean lawmakers.

Deputy Inspector-General of police Noor Rashid Ibrahim told a news conference that Malaysia was coordinating with Interpol to track down the four North Koreans, but would not reveal where they flew to on the day of the murder.

"The four suspects are holding normal passports, not diplomatic passports," he said. "Next plan is to get them. We of course have international cooperation especially with Interpol, bilateral involvement with the country involved, we will go through those avenues to get the people involved."

The four suspects arrived in Malaysia just days before the attack on Kim Jong-nam, according to police.

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Mr Noor Rashid named the four who escaped as Ri Ji-hyon, Hong Song-hac, O Joong-gil, and Ri Jae-nam. The police are looking for three other people who are not suspects but who they believe could help with their enquiries, one of whom is North Korean.

Police said the cause of death was still not known and that they were waiting for pathology and toxicology tests after conducting a post-mortem.

Police believe two women attacked Kim Jong-nam at about 9am on Monday in the departure hall of the budget terminal of Kuala Lumpur International Airport.

Noor Rashid said the victim complained to the customer service desk that two women had "wiped his face with a liquid". Police are awaiting the autopsy report to clarify what that liquid was.

Kim Jong-nam was taking a flight back to Macau and was in Kuala Lumpur on "personal business", police added.

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The mother of the Indonesian woman told Reuters that her daughter, Siti Aishah, had been duped into believing she was part of a TV show or advertisement.

"She said she wanted to go to Malaysia for filming on a show to make people surprised by spraying perfume on somebody else," said Benah, who goes by one name. "She was offered a job by someone to become an advertisement model for perfume."

Asked about the theory that the women thought they were playing a prank, Mr Noor Rashid said: "I think there is footage being released ... and you can interpret from the footage."

South Korea's intelligence agency told lawmakers in Seoul that Kim Jong Nam had been living with his second wife in the Chinese territory of Macau, under China's protection.

North Korea has said it will reject Malaysia's autopsy report and accused Malaysia of "colluding with outside forces", a veiled reference to rival South Korea.

"They can say anything, but as far as we are concerned we follow legal and procedural requirements of our country," Mr Noor Rashid said.

He added that police were trying to contact Kim Jong-nam's next-of-kin to identify the body through "scientific means", and would give them two weeks to claim the body.

Malaysia is one of the few countries that has maintained good diplomatic relations with North Korea.

Pyongyang's nuclear arms and weapons programmes have alarmed the West, most recently its test of a ballistic missile earlier this month in its first direct challenge to the international community since Donald Trump became US president.

Pyongyang's main ally and trading partner is China, which is irritated by its repeated aggressive actions but rejects suggestions from the United States and others that it could be doing more to rein in its neighbour.

On Saturday, China said it had further tightened trade restrictions with North Korea by suspending all imports of coal starting 19 February, although it did not say why. Coal exports to China are a vital source of revenues for Pyongyang.