South Korea said on Wednesday it believed the half-brother of North Korean leader Kim Jong Un had been murdered, as Malaysian authorities moved the body to another hospital for an autopsy to identify the cause of death, sources told Reuters.

U.S. government sources told Reuters they believed that Kim Jong Nam, who according to Malaysian police died on Monday on his way to hospital from Kuala Lumpur International Airport, had been murdered, possibly poisoned, by North Korean agents.

Kim Jong Nam had spoken out publicly against his family's dynastic control of the isolated state and was estranged from the young North Korean leader.

"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, the country's acting president, told a security council meeting.

South Korea is acutely sensitive to any sign of potential instability in North Korea, and is still technically in a state of war with its impoverished and nuclear-armed neighbour.

TICKET TO MACAU

Malaysian police said the dead man, 46, held a passport under the name Kim Chol. Kim Jong Nam was known to spend a significant amount of time outside North Korea, and has been caught in the past using forged travel documents.

Malaysian police official Fadzil Ahmat said on Tuesday the cause of Kim's death was not yet known, and that a post-mortem would be carried out. Kim had been planning to travel to Macau on Monday when he fell ill at KLIA's low-cost terminal, Fadzil said.

South Korea's intelligence agency said Kim's family was in Macau, South Korea's Yonhap News Agency reported.

Asked during a news briefing if the murder of Kim Jong Nam was confirmed, South Korean Unification Ministry spokesman Jeong Joon-hee said: "Yes, I have said it is confirmed."

South Korea's TV Chosun, a cable-TV network, cited multiple South Korean government sources saying that Kim had been poisoned with a needle by two women believed to be North Korean operatives who fled in a taxi.

Reuters could not independently confirm those details.

The North Korean embassy in Kuala Lumpur would not speak to reporters gathered outside its gate and refused them entry. A few cars were seen leaving the embassy.

South Korea's Unification Ministry urged North Korean defectors in South Korea and abroad to be mindful of their security.