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 North Korean assassins killed Kim Jong Nam. Malaysian police said he had been assaulted on Monday in Kuala Lumpur International Airport and died on the way to hospital.

South Korean intelligence believed Kim Jong Nam was poisoned, lawmakers said after being briefed by the 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.

Kim had been at the airport's low-cost terminal to catch a flight to Macau on Monday, when someone grabbed or held Kim's face from behind, after which he felt dizzy and sought help, Malaysian police official Fadzil Ahmat told Reuters.

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

Portly and gregarious, Kim Jong Nam was the eldest son of late North Korean leader Kim Jong Il and 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, who is also acting president, told a security meeting.

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

South Korea is acutely sensitive to any sign of instability in isolated North Korea, and is still technically in a state of war with its impoverished and nuclear-armed neighbour, which carried out its latest ballistic missile test on Sunday.

TICKET TO MACAU

Malaysian police said the dead man held a passport under the name Kim Chol, with a birth date that made him 46.

Kim Jong Nam was known to spend a significant amount of time outside North Korea, travelling in Macau and Hong Kong as well as mainland China, and has been caught in the past using forged travel documents.

His body was taken on Wednesday morning to a second hospital, where an autopsy was being performed. North Korean embassy officials had arrived at the hospital and were coordinating with local authorities, police sources said.

One of the South Korean lawmakers said Seoul's spy agency expected the body would be returned to Kim's family in Macau.

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."

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.

There was no mention of Kim Jong Nam's death in North Korean state media.

In Beijing, China's Foreign Ministry spokesman Geng Shuang told a daily news briefing that he noted media reports of Kim's death and understood the Malaysian authorities were investigating.

WHOSE ORDERS?

Michael Madden, a U.S.-based expert on the North Korean leadership, cast doubt on the notion that Kim Jong Un had personally ordered the killing of his half-brother.

Doing so would further feed the perception that Kim Jong Un was engaged in a "reign of terror" and is insecure about his leadership, and would also irritate China and Malaysia, two of the few countries with which North Korea has relatively good relations, he said.

"It does not serve Kim Jong Un's political interests to have Jong Nam assassinated," Madden said. "It is likely that if he was killed by North Korean operatives, then someone else pushed the button."

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

Numerous North Korean officials have been purged or killed since Kim Jong Un took power following his father's death in 2011. Those include his uncle Jang Song Thaek, who was considered the country's second most-powerful person and was believed to have been close to Kim Jong Nam.

Jang was executed on Kim Jong Un's orders in 2013.

Koh Yu-hwan, a professor of North Korean studies at Dongguk University in Seoul, said that given Kim Jong Nam's family connection, it was "difficult to imagine" an assassination would be carried out without the leader's consent.

"Kim Jon Un may have been worried about more and more of North Korean elites turning against him after Thea Yon Ho defected to the South," he said, referring to last year's defection by North Korea's deputy ambassador in London.