Kim Jong-Nam was reportedly murdered by two female Pyongyang agents at the capital’s international airport on Monday.
The women used poisoned needles to kill him and immediately fled the scene in a taxi, according to the South Korean broadcaster TV Chosun, citing Seoul government sources.
But a Malaysian official claimed he had been sprayed with a liquid in a shopping area and, complaining of pain, he tried to get help at an information counter before being taken to the airport clinic.
The country’s police confirmed the 45-year-old was due to catch a flight to Macau, but was found ill at the airport and died on his way to hospital. Officers are investigating the cause of death.
In a statement, Malaysian officers said the victim held a passport under the name Kim Chol.
Kim Jong-Nam is the son of Song Hye-Rim, a South Korean-born actress who is believed to have been a mistress of former North Korean leader Kim Jong-Il.
He was a known advocate of reform in North Korea and was vocal about his opposition to the nation’s dynastic succession policy.
Understood to be Kim Jong-Il’s heir apparent as his eldest son, he was overlooked for the succession after a botched attempt to enter Japan in May 2001 on a false Dominican Republic passport.
He claimed he wanted to visit Disney’s Tokyo resort, but the incident embarrassed his father, and the then North Korean leader was succeeded by Kim Jong-Un in December 2011.
Kim Jong-Un has an older brother Kim Jong-chul, an older half-sister Kim Sul-song and a younger sister Kim Yo-jong.
Following the Japan scandal, Kim Jong-Nam and his family virtually lived in exile in Macau, Singapore and China.
According to South Korean prosecutors, a North Korean detained as a spy in October 2012 admitted being involved in a 2010 plot to stage a hit-and-run car accident targeting Kim Jong-Nam in China.
His son Kim Han-Sol, who studied in Paris, once described Kim Jong-Un, his uncle, as a “dictator” in an interview.
His father’s death comes amid attempts by the North Korean leader to strengthen his grip on power in the face of growing international condemnation of Pyongyang’s nuclear and missile programmes.
In December 2013, the North Korean leader’s uncle was branded a “traitor” and executed by machine gun for “attempting to overthrow the state by all sorts of intrigues and despicable methods”.
Once considered the second most powerful official in the North, General Jang Song-Thaek was believed to have helped Mr Kim consolidate power after the death of his father.