South Korea's spy agency suspects two female North Korean agents assassinated the estranged half-brother of North Korean leader Kim Jong-un in Malaysia, lawmakers in Seoul said on Wednesday, as Malaysian medical authorities sought a cause of death.

U.S. government sources also 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 by North Korean assassins.

South Korean intelligence believed Kim Jong-nam was poisoned, lawmakers said after being briefed by the country's spy agency.

They said the spy agency told them that the young, unpredictable North Korean leader had issued a "standing order" for his half-brother's assassination, and that there had been a failed attempt in 2012.

According to the spy agency, Kim Jong-nam had been living with his second wife in the Chinese territory of Macau, under Beijing's protection, the lawmakers said. One of them said Kim Jong-nam also had a wife and son in Beijing.

Kim Jong-nam and Kim Jong-un are both sons of former North Korean leader Kim Jong-il, who died in 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 the relationship a secret.

NORTHKOREA-MALAYSIA/KIM

Police officers early Wednesday stand outside the Kuala Lumpur Hospital, where the body of Kim Jong-nam is held for autopsy, in Putrajaya, Malaysia. (Edgar Su/Reuters)

Portly and gregarious, Kim Jong-nam had spoken out publicly against his family's dynastic control of the isolated state.

"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.

The meeting was called in response to Kim Jong-nam's mysterious death, news of which first emerged late on Tuesday. 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.

Closed-circuit footage tracks woman

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, traveling in Macau and Hong Kong as well as mainland China, 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.

Kim's body was taken on Wednesday morning to a second hospital, where an autopsy was being performed.

North Korea Kim's Uncle

Kim Jong-nam was believed to be close to his uncle, Jang Song Thaek, seen here, on the right, with North Korean leader Kim Jong-un in 2013. Jang was executed for treason later that year. (Kyodo News/Associated Press)

A Malaysian police source who had seen closed-circuit TV footage from the airport said a woman was involved in the attack.

"So far from the CCTV we can confirm it's a woman," the source said.

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.

Officials at 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.

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

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

With files from The Associated Press