The source declined to reveal the exact poison used but described it as "more potent than cyanide".
S.Korea: Fingerprints confirmed as Kim Jong Nam's
South Korean government officials say fingerprint checks show it's certain the man who died in Malaysia was Kim Jong Nam, half-brother of North Korea's leader Kim Jong Un.
The South Korean officials told NHK that the Malaysian government asked them on Tuesday to compare fingerprints taken from the body with those of Kim Jong Nam, which they hold. The officials said they confirmed the prints match.
On Monday, a man with North Korean citizenship fell ill at Kuala Lumpur International Airport. He died while being taken to a hospital.
South Korea's intelligence officials said on Wednesday that they believe he was killed with poison by agents of North Korea.
They also said the act was likely done with Kim Jong Un's approval.
Malaysian police are trying to determine the cause of the death and are interrogating a woman with a Vietnamese passport detained in connection with the suspected killing.
Reuters news agency quoted sources close to Malaysian government officials as saying North Korean officials opposed an autopsy on the body and requested its immediate handover. The sources said Malaysia rejected the request.