Malaysian police say a woman has been arrested at Kuala Lumpur International Airport in connection with the death of Kim Jong-un's estranged half-brother.
They said the suspect was carrying a Vietnamese travel document and was alone at the time of arrest.
South Korea's spy agency suspects two female North Korean agents assassinated Kim Jong-nam in Malaysia, politicians in Seoul said overnight, as Malaysian medical authorities sought a cause of death.
US 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, politicians said after being briefed by the country's spy agency.
North Korean officials spent hours today trying to talk Malaysia out of conducting a post-mortem examination Kim Jong-nam, according to sources.
Malaysian authorities refused the request, said the sources, all Malaysian government officials.
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 politicians said.
One of them said Kim Jong-nam also had a wife and son in Beijing.
He 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 yesterday.
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.
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, travelling 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 the cause of death was not yet known, and that a post-mortem would be carried out.
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."