Officials in Seoul and Kuala Lumpur could not confirm the death of Kim Jong-Nam, once seen as heir apparent in the North.
Malaysian police said in a statement late Tuesday that a North Korean man, identified as Kim Chol, sought medical assistance at Kuala Lumpur International Airport and died on the way to hospital.
South Korean media reports meanwhile said Jong-Nam had travelled using a fake passport under the name of Kim Chol.
If confirmed, it would be the highest-profile death under the Jong-Un regime since the execution of the leader's uncle Jang Song-Thaek in December 2013.
Jong-Un has been trying to strengthen his grip on power in the face of growing international pressure over his country's nuclear and missile programmes. He has reportedly staged a series of executions.
The latest launch of a new intermediate-range missile on Sunday brought UN Security Council condemnation and vows of a strong response from US President Donald Trump.
South Korea's national news agency Yonhap quoted a source as saying agents of the North's spy agency, the Reconnaissance General Bureau, carried out the assassination on Monday by taking advantage of a security loophole between Jong-Nam's bodyguards and Malaysian police at the airport.
