During a meeting with the National Assembly's Intelligence Committee, Lee confirmed that Kim was killed with poison at the airport, although it still needs to find out whether a needle or chemical spray was used. The agency conceded it was notified of the incident about four hours after it occurred.

Kim was at the airport to take an airplane heading to Macao when he asked the staff for help after interacting with two "Asian" women. Kim died on the way to a nearby hospital.

The Malaysian authorities presume Kim was poisoned, although details will be revealed through an autopsy. The agency said the suspects are presumed to be still at large in Malaysia.

"There was also an (assassination attempt) in 2012," Lee was quoted as saying by Rep. Kim Byung-kee of the Democratic Party.

Lee then said the North's latest action is presumed to have been based on Kim Jong-un's "delusional disorder," rather than on any calculation that his half brother is a threat to the regime.

The NIS head also told lawmakers that Kim sent a letter to the North Korean leader Kim Jong-un in 2012, asking his brother to spare his and the lives of his family.

Kim Jong-nam's wife is currently staying in Beijing with a son, while his second wife is living in Macao with a son and a daughter.

All of them are currently under the protection of Chinese authorities.

The son in Macao is Kim Han-sol, who came into the spotlight in 2012 after being interviewed by a Finnish television station. He said he hopes for the unification of the two divided Koreas and wants to improve the livelihoods of ordinary people living in the impoverished country now controlled by his young uncle.

The agency said Kim has never asked South Korea for asylum, nor was there an effort within North Korea to place Kim Jong-nam as the leader.

South Korea's unification ministry also said it is certain that Kim has been killed in Malaysia.

"Police in Malaysia are conducting a probe into the killing, and they have yet to announce the results (of the investigation)," Jeong Joon-hee, spokesman at Seoul's unification ministry said.

"Seoul is closely cooperating with the Malaysian government."

Experts said that Kim might have been killed due to the North Korean leader's move to strengthen his reign of terror by eliminating any potential rivals to his iron-fist rule.

Kim Jong-nam -- the eldest son of late former leader Kim Jong-il -- had been living in foreign countries for years after apparently falling out of favor with his father for attempting to enter Japan with a fake passport in 2001.

He was critical of the power succession to his brother Kim Jong-un. In 2010, Kim Jong-nam told Japan's TV Asahi that he is "against third-generation succession," although he said he hopes Kim Jong-un will do his best to improve the lives of North Koreans and that he stands ready to help from abroad.

It marks the highest-profile death under the Kim Jong-un regime since the execution of Jang Song-thaek in December 2013, the once-powerful uncle of the incumbent leader.

Since inheriting power from his father Kim Jong-il in late 2011, the North's young leader has propped up his control by executing more than 100 military, party and government officials, according to Seoul's spy agency.

Japan's Kyodo News, meanwhile, said that authorities in Tokyo are checking reports that the two suspected assailants may have died.

South Korea police said it has increased the number of officers protecting high-ranking North Korean defectors in the country.

"We have added more forces to the team since Tuesday night amid the rising concerns over the safety of many important defectors," an official from the police said. (Yonhap)