The US government strongly believes that North Korean agents murdered the estranged half-brother of North Korean leader Kim Jong-un in Malaysia, US government sources said yesterday.

American authorities have not yet determined exactly how Kim Jong-nam was killed, according to two sources, who did not provide specific evidence to support the US government's view.

A South Korean government source also had said that Kim Jong-nam had been murdered in Malaysia. He did not provide further details.

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.

In Washington, there was no immediate response to a request for comment from the Trump administration, which faces a stiff challenge from a defiant North Korea over its nuclear arms programme and the test of a ballistic missile last weekend.

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.

In a statement, Malaysian police said the dead man, 46, held a passport under the name Kim Chol.

Kim Jong-nam has been caught in the past using forged travel documents.

Malaysian police official Fadzil Ahmat said the cause of Kim's death was not yet known, and that a post-mortem would be carried out.

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

According to Fadzil, Kim had been planning to travel to Macau on Monday when he fell ill at the low-cost terminal of the KL International Airport (KLIA).

"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.

The US government sources said it was possible that Kim Jong-nam had been poisoned. They said it could not be ruled out that assassins used some kind of "poison pen" device.

South Korea's TV Chosun, a cable-TV 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.

Reuters could not independently confirm those details.

Secretive family

Malaysia is one of a dwindling number of countries that has close relations with North Korea, which is under tightening global sanctions over its nuclear tests and ballistic missile launches, the latest of which took place on Sunday.

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

A phone call to the North Korean embassy in Kuala Lumpur late yesterday went straight to an answering machine.

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, the elder of the two, did not attend his father's funeral. His mother was an actress named Song Hye-rim, and Kim Jong-nam said his father kept his parents' relationship secret.

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.

In an embarrassing 2001 incident, Kim Jong-nam was caught at an airport in Japan travelling on a forged Dominican Republic passport, saying he had wanted to visit Tokyo Disneyland. He was known to travel to Hong Kong, Macau and mainland China.

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.

- Reuters