The elder half-brother of Kim Jong-un has been assassinated in a Malaysian airport, with the finger being pointed at agents of the North Korean leader.

Kim Jong-nam told medical workers he had been grabbed from behind and attacked with a chemical spray, according to a Malaysian official.

Multiple South Korean media reports said he had been assassinated by two female North Korean agents who had fled in a taxi and were being sought by Malaysian police.

South Korea's TV Chosun, a cable-TV network, reported that Kim had been poisoned with a needle, citing multiple South Korean government sources.

Details uncertain, but North Korea strongly suspected

Kim Jong-un (R) has a history of assassinating those perceived as a threat to his authority, including his uncle Jang Song-thaek (L) ( Reuters/Kyodo )

US Government sources said they strongly believed North Korean agents murdered Kim Jong-nam.

American authorities have not yet determined exactly how he was killed but it could not be ruled out that assassins used some kind of "poison pen" device, according to two sources.

There was no immediate response to a request for comment from the Trump administration.

South Korea's foreign ministry said it could not confirm the reports, and the country's intelligence agency could not immediately be reached for comment.

Kim Jong-nam was known to spend a significant amount of his time outside North Korea and had spoken out publicly against his family's dynastic control of the isolated state.

If confirmed as an assassination, it would be the latest in a string of killings over the decades at home and abroad meant to silence those perceived by North Korea's leaders as threats to their authority, one of the US sources said on condition of anonymity.

Kim Jong-nam rejected power, but speculation continued

Kim Jong-nam was being considered by China as a successor to Kim Jong-un. ( AP: Shin In-seop/JoongAng Ilbo, File )

Kim Jong-nam and Kim Jong-un are both sons of former North Korean leader Kim Jong-il, who died in late 2011, but they had different mothers.

The portly and easy-going Kim Jong-nam, the elder of the two, was believed to be close to his uncle, Jang Song-thaek, who was North Korea's second most powerful man before he was executed on Kim Jong-un's orders in 2013.

Koh Yu-hwan, a professor at Dongguk University in Seoul, said Kim Jong-nam had occasionally been the subject of speculation that he could replace his younger half-brother, the country's third-generation leader.

"Loyalists may have wanted to get rid of him," he said.

Kim Jong-nam said several times over the years that he had no interest in leading his country.

"Personally, I am against third-generation succession," he told Japan's Asahi TV in 2010. "I hope my younger brother will do his best for the sake of North Koreans' prosperous lives."

His cousin, Lee Han-young, who defected to South Korea through Switzerland in 1982, was shot and killed by North Korean agents in Seoul in 1997, according to South Korea.

Assassination looks like Kim Jong-un asserting his authority

Kim Jong-nam's death does look like an assassination of a high-ranking North Korean, said Leonid Petrov from the ANU College of Asia and the Pacific.

"I would say timing is crucial here. North Korea is celebrating the 75th anniversary of North Korea's late leader Kim Jong-il, and many see this as Kim Jong-un asserting control," he told ABC News Breakfast.

Dr Petrov said that although Kim Jong-nam was widely assumed to have no interest in gaining power, North Korea was a very traditional society where it is important to bequeath to the eldest song, rather than the youngest.

Sorry, this video has expired Dr Leonid Petrov says Kim Jong-nam was living a risky life and causing reputational damage to Kim Jong-un.

"So there could be bad blood between the brothers," he said.

Kim Jong-nam was also known for business activity in South-East Asia and it is possible he was assassinated by gang activity, Dr Petrov said.

However, North Korea's neighbours had plans for Kim Jong-nam and Kim Jong-un may have felt threatened.

"I think that Kim Jong-nam, while he was alive, was causing much more reputational damage to Kim Jong-un in the family," Dr Petrov said.

"I know that China was actually considering him as a potential successor to the dynasty if something happened to Kim Jong-un.

"I don't think he would be very popular, but alternatively as a figure who could bring some change, reform, market-oriented reform in North Korea would be much welcomed by China … I think Japan would support it, too."

'It all smacks of the James Bond-type method'

Aidan Foster-Carter from Leeds University said he was "gobsmacked" by Kim Jong-nam's death and although "there are reasons why this might have happened, I certainly wasn't expecting it now".

"We hadn't heard from him for some time and I assumed he was lying low and that he wasn't really a threat to anybody," Mr Foster-Carter said.

Sorry, this video has expired Aidan Foster-Carter says the assassination looks like something planned by North Korea.

It would seem that Kim Jong-un thought differently, Mr Foster-Carter said, adding that although the North Korean dictator had inherited the position while young, he looked to be hunkering down and consolidating.

"I can see why he might have thought [Jang Song-thaek] was a threat. Kim Jong-nam, I'm surprised at that. But as I say, maybe, uneasy lays the head that wears the crown," Mr Foster-Carter said.

Mr Foster-Carter also said gang activity is a possibility, saying it is conceivable Kim Jong-nam had gambling debits.

"But then it would have been a quick bullet, wouldn't it, or something like that," he said.

"It all smacks a bit of the James Bond-type method, which North Korea's slightly famous for."

ABC/wires