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.

US 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."