Two women and a man have been arrested over the killing of the North Korean leader's half brother, who was reportedly poisoned by a pair of female assassins as he waited for a flight in Malaysia, Malaysian police said.

Investigators are trying to piece together the details of a death that set off a torrent of speculation over whether Kim Jong Un dispatched a hit squad to kill his estranged older sibling, Kim Jong Nam.

The suspects were picked up separately on Wednesday and Thursday. The women were identified using surveillance footage from Kuala Lumpur International Airport, where Kim Jong Nam fell ill on Monday morning before dying on the way to hospital.

One of the women had Vietnamese travel documents and the other held an Indonesian passport.

Indonesia's foreign ministry said one woman is an Indonesian citizen and officials have requested consular access to her.

Malaysian police identified her as Siti Aishah, 25, from Serang in Banten, a province that neighbours Indonesian capital Jakarta.

A still photo from airport surveillance video showed one of the suspects in a white T-shirt with "LOL" across the front.

On Thursday afternoon, police said they had detained a Malaysian man believed to be the boyfriend of the suspect with an Indonesian passport.

A post-mortem examination was finished late on Wednesday, but the results have not been released. The findings could reveal whether Kim Jong Nam was poisoned, and possibly shed light on the tales of intrigue that have rippled since his death.

North Korea had objected to the post-mortem but Malaysia went ahead as the North did not submit a formal protest, said Abdul Samah Mat, a senior police official.

Deputy home minister Zahid Hamidi said security is the top priority for the government. Asked at a news conference why Malaysia failed to protect Kim Jong Nam, he said: "What do you mean? Do we have to engage a bodyguard and usher him everywhere? No."

Kim Jong Nam, who was 45 or 46, was estranged from his North Korean relatives and had been living abroad for years.

He reportedly fell out of favour with his father when he was caught trying to enter Japan on a false passport in 2001, saying he wanted to visit Tokyo Disneyland. That episode and his years abroad were seen as souring his leadership potential.

According to two senior Malaysian government officials, he told medical workers before he died that he had been attacked with a chemical spray. South Korean media reports, citing unidentified sources, said two women believed to be North Korean agents killed him with some kind of poison before fleeing in a taxi.

Since taking power on his father's death in late 2011, Kim Jong Un has executed or purged a number of high-level government officials in what the South Korean government has described as a "reign of terror".

South Korea's spy agency, the National Intelligence Service, said on Wednesday that North Korea had been trying for five years to kill Kim Jong Nam.