South Korea's spy agency said on Wednesday that Kim Jong-nam, the estranged half-brother of North Korean leader Kim Jong-un, was poisoned by two suspected North Korean female agents, according to MPs briefed by the intelligence agency.

The agency also said North Korea had long prepared for the murder of Kim Jong-nam, the MPs said, although they did not explain how the agency knows that.

Kim fell suddenly ill at an airport in Malaysia's Kuala Lumpur on his way to the Chinese territory of Macau, where he had been living, the MPs said, adding that he had been given China's protection.

He died on his way to hospital.

Police reports said the two female agents fled in a taxi and were being sought by Malaysian police.

Malaysian authorities detained a woman from Myanmar in connection with the investigation, Malaysian state news agency Bernama reported.

Meanwhile, police were checking surveillance tapes for more clues over his death, a senior official said.

An autopsy will also be performed on the body of Kim Jong-nam on Wednesday, police said.

"If the murder of Kim Jong-nam was confirmed to be committed by the North Korean regime, that would clearly depict the brutality and inhumanity of the Kim Jong-un regime," South Korean Prime Minister Hwang Kyo-ahn, who is also the country's acting president, told a security council meeting.

Al Jazeera's Harry Fawcett, reporting from Seoul, said: "The National Intelligence Service here says this is a long-standing order from the North Korean leadership. There was [reportedly] an attempt in 2012 to kill him. There have long been efforts to eradicate this family member."

While there was speculation over the timing of the incident, the head of spy agency has said that North Korea probably just made the most of an opportunity that presented itself, said Fawcett.

'Chemical spray'

Kim, 46, was targeted Monday in the shopping concourse at the airport and had not yet gone through immigration for his flight to Macau, said a senior government official, who spoke on condition of anonymity because the case involves sensitive diplomacy.

Kim told medical workers that he had been attacked with a chemical spray, the official said.

Although Kim had been tipped by some outsiders as a possible successor to his father, others thought that was unlikely because he lived outside the country, including recently in Macau, Singapore and Malaysia.

OPINION: Trouble for Trump in North Korea?

He reportedly fell further out of favour with the current North Korean leader when he was caught trying to enter Japan on a false passport in 2001, saying he wanted to visit Tokyo Disneyland.

A Malaysian police statement confirmed the death of a 46-year-old North Korean man whom it identified from his travel document as Kim Chol, born in Pyongyang on June 10, 1970.

"Investigation is in progress and a post-mortem examination request has been made to ascertain the cause of death," the statement said.

Kim Jong Un's 'paranoia'

Ken Gause, who has studied North Korea's leadership for 30 years at the CNA think-tank in Washington, said Kim Chol was a name that Kim Jong-nam has travelled under.

He is believed to have been born May 10, 1971, although birthdays are always unclear for senior North Koreans, Gause said.

Mark Tokola, vice president of the Korea Economic Institute in Washington and a former deputy chief of mission at the US embassy in Seoul, said it would be surprising if Kim was not killed on the orders of his brother, given that North Korean agents have reportedly tried to assassinate Kim in the past.

"It seems probable that the motivation for the murder was a continuing sense of paranoia on the part of Kim Jong-un," Tokola wrote in a commentary on Tuesday.

Although there was scant evidence that Kim Jong-nam was plotting against the North Korean leader, he provided an alternative for North Koreans who would want to depose his brother.

The reported killing came as North Korea celebrated its latest missile launch, which foreign experts were analysing for evidence of advancement in the country's missile capabilities.

READ MORE: North Korea declares ballistic missile test 'success'

For the next several days, North Korea will be marking the birthday of its late leader Kim Jong-il, the brothers' father, though they have different mothers.

The major holiday this Thursday is called the "Day of the Shining Star" and will be feted with figure skating and synchronised swimming exhibitions, fireworks and mass rallies.

Since taking power in late 2011, Kim Jong-un has executed or purged a slew of high-level government officials in what the South Korean government has described as a "reign of terror".

The most spectacular was the 2013 execution by anti-aircraft fire of his uncle, Jang Song-thaek, once considered the country's second-most powerful man, for what the North alleged was treason.

Gause said Kim Jong-nam had been forthright that he did not have political ambitions, although he was publicly critical of the North Korean regime and his brother's legitimacy in the past.