The estranged half-brother of North Korean leader Kim Jong-un has been murdered in Malaysia, a South Korean government source says.

Kim Jong-nam, the older half brother of the North Korean leader, was known to spend a significant amount of his time outside the country and had spoken out publicly against his family's dynastic control.

He was confirmed dead by Malaysian police, and was believed to be in his mid- 40s.

Police official Fadzil Ahmat said that the cause of Kim's death had not been determined yet, but that a post mortem would be carried out on the body.

According to Fadzil, Kim had been planning to travel to Macau on Monday when he fell ill at the Kuala Lumpur International Airport.

"The deceased ... felt like someone grabbed or held his face from behind," Fadzil said. "He felt dizzy, so he asked for help at the ... counter of KLIA."

Kim was taken to an airport clinic where he still felt unwell, and it was decided to take him to hospital. He died in the ambulance on the way to Putrajaya Hospital, Fadzil added.

South Korea's TV Chosun, a cable television network, reported that Kim had been poisoned with a needle by two women believed to be North Korean operatives who fled in a taxi and were at large, citing multiple South Korean government sources.

Malaysians and North Koreans can visit each other's country without visas.

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.

Kim Jong-nam did not attend his father's funeral.

The portly and easygoing Kim Jong-nam was believed to be close to his uncle, Jang Song-thaek, who was North Korea's second most powerful man before being executed on Kim Jong-un's orders in 2013.

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