Officials in Seoul could not be reached for comment on the reported death.
Officials in Seoul could not be reached for comment on the reported death of Kim Jong-Nam. Malaysian police said an unidentified Korean had been taken sick at Kuala Lumpur International Airport and since died.
If confirmed, Kim's case would be the highest-profile death under the Kim Jong-Un regime since the execution of the leader's uncle Jang Song-Thaek in December 2013.
Kim Jong-Un has been trying to strengthen his grip on power in the face of growing international pressure over North Korea's nuclear and missile programmes.
The latest missile launch on Sunday brought UN Security Council condemnation.
South Korea's national news agency Yonhap quoted a Seoul government source as saying Kim Jong-Nam was killed on Monday. The source gave no further details.
The 45-year-old was poisoned by two unidentified female agents using poisoned needles at an airport in Kuala Lumpur, according to South Korean broadcaster TV Chosun.
The report, citing what it called multiple government sources, said the two women hailed a cab and fled immediately afterwards.
In Malaysia, the police chief in charge of Kuala Lumpur International Airport, Assistant Commissioner Abdul Aziz Ali, told AFP a Korean in his forties was found sick at the airport on Monday.
Airport authorities rushed him to the hospital and he died on the way, the police chief said.
"We do not have any other details of this Korean man. We do not know his identity," Abdul said.
Kim Jong-Nam was once considered heir apparent but fell out of favour with his father Kim Jong-Il following an embarrassing botched attempt in 2001 to enter Japan on a forged passport and visit Disneyland.
He has since lived in virtual exile, mainly in the Chinese territory of Macau.
Jong-Nam's half-brother Jong-Un took over as North Korean leader when their father died in December 2011.
Jong-Nam, known as an advocate of reform in the North, once told a Japanese newspaper that he opposed his country's dynastic power transfers.
He was reportedly close to his uncle Jang Song-Thaek, once the North's unofficial number two and political mentor of the current leader.
Jong-Nam has been targeted in the past. In October 2012 South Korean prosecutors said a North Korean detained as a spy had admitted involvement in a plot to stage a hit-and-run car accident in China in 2010 targeting Kim Jong-Nam.
In 2014 Jong-Nam was reported to be in Indonesia -- sighted at an Italian restaurant run by a Japanese businessman in Jakarta -- and was said to be shuttling back and forth between Singapore, Indonesia, Malaysia and France.
In 2012 a Moscow newspaper reported that Jong-Nam was having financial problems after being cut off by the Stalinist state for doubting its succession policy.
The Argumenty i Fakty weekly said he was kicked out of a luxury hotel in Macau over a $15,000 debt.
Last year South Korea warned of possible North Korean assassination attempts on its territory.
It noted previous attempts to assassinate Hwang Jang-Yop, the North's chief ideologue and former tutor to Kim Jong-Il, who defected to the South in 1997 and died of natural causes in 2010.
Jong-Nam was born from his father's extra-marital relationship with Sung Hae-rim, a South Korean-born actress who died in Moscow.