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 neighbor, which carried out its latest ballistic missile test on Sunday.