The half-brother of North Korean leader Kim Jong-un has been assassinated at an airport in Kuala Lumpur, telling medical workers before he died that he was attacked with a chemical spray, a Malaysian official has said.

Key points: Kim Jong-nam reportedly killed while travelling under a pseudonym he has used before

Kim Jong-nam reportedly killed while travelling under a pseudonym he has used before Told medical staff he was "grabbed from behind" and attacked with a chemical spray

Told medical staff he was "grabbed from behind" and attacked with a chemical spray Two women believed to be North Korean agents are being sought by Malaysian police

Kim Jong-nam, 46, was targeted on Monday in the shopping concourse at Kuala Lumpur International Airport (KLIA) and had not gone through immigration yet for his flight to Macau, said the senior government official, who spoke on condition of anonymity.

Multiple South Korean media reports said Kim Jong-nam was poisoned, allegedly by two female North Korean assassins.

TV Chosun, citing "multiple government sources," said the women were believed to be North Korean agents. It said they fled in a taxi and were being sought by Malaysian police.

Kim Jong-nam was taken to the airport clinic and then died on the way to the hospital, police official Fadzil Ahmat said.

"The deceased … felt like someone grabbed or held his face from behind," Mr Fadzil said.

"He felt dizzy, so he asked for help at the … counter of KLIA.

"So far there are no suspects, but we have started investigations and are looking at a few possibilities to get leads."

He said the cause of death was not yet known, and a post mortem would be carried out on the body.

In a statement, Malaysian police said the dead man, aged 46, held a passport under the name Kim Chol, which said he was born in Pyongyang on June 10, 1970.

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

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

Security personnel at the hospital Kim Jong-nam was taken to after collapsing at an airport. ( AP: Vincent Thian )

Once tipped to succeed Kim Jong-il but undone by trip to Tokyo Disneyland

The incident 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.

Sorry, this video has expired Dr Leonid Petrov says Kim Jong-un may be asserting control during anniversary celebrations for their father Kim Jong-il.

Kim Jong-nam is the eldest son of former North Korean leader, the late Kim Jong-il, who was also the father of the current leader.

He had been tipped by outsiders to succeed their dictator father, but reportedly fell out of favour when he was caught trying to enter Japan on a false passport in 2001, saying he wanted to visit Tokyo Disneyland.

He was believed to have been living recently in Macau, Singapore and Malaysia, and it is unclear why Kim Jong-nam would be assassinated, Dr Petrov said.

"It's widely assumed that he didn't want to in any way try to go back and take power. He was out of North Korea."

In Washington, the State Department said it was aware of reports of Kim Jong-nam's death but declined to comment, referring questions to Malaysian authorities.

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

For the next several days, North Korea will be marking the birthday of its late leader Kim Jong-il. The major holiday this Thursday is called the "Day of the Shining Star" and will be feted with figure skating and synchronized 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.

Kim Jong-nam was the eldest son of Kim Jong-il, but left North Korea in 2001. ( AP: Shizuo Kambayashi, file photo )

AP/Reuters