"If it is confirmed, this would be the result of Kim Jong-un's paranoia against Kim Jong-nam," he said. "In the early 1990s, North Korea had started efforts to make Kim Jong-nam nobody."

Kim Jong-nam, the eldest son of late former leader Kim Jong-il, died last week at an airport in Malaysia after being attacked by two Asian women with what appeared to be a poisonous substance.

"Even if North Korea denies any allegation, the North elite would think 100 percent that the (assassination) was ordered by Kim Jong-un," Thae claimed.

He said that the incident will likely deal a heavy blow to the North Korean regime over the long term as it serves as material evidence to back up North Korea's crimes against humanity.

Thae's remark came as Malaysian police said Sunday that at least five North Korean suspects are involved in the incident.

Seoul said that it is highly likely that North Korea is behind the killing, given that Pyongyang has a track record of committing terrorist attacks and inhumane acts.

Kim Jong-nam 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.

In a patriarchal society like North Korea, the first son is widely viewed as the successor, but the power succession was given to Kim Jong-un, who is the third son. He assumed power in late 2011 following the sudden death of his father.

For the current leader, this is seen as an Achilles' heel in terms of the legitimacy of his leadership, along with the uncertainty over the background of his late mother Ko Yong-hui, who was a Korean resident in Japan. Korean residents in Japan are viewed as having a low societal status in the North.

"The existence of Kim Jong-nam is a hindrance in making North Koreans recognize Kim Jong-un's legitimacy as royal blood," Thae said.

The latest killing has also raised safety concerns for North Korean defectors, making the government beef up security for high-profile defectors including Thae.

Rep. Ha Tae-Keung of the splinter Bareun Party claimed last week that two North Korean spies have entered South Korea to attack defectors, warning that Thae would be the No. 1 target. He did not reveal where he obtained the intelligence.

Thae, a former North Korean diplomat in London who escaped to Seoul with his family in late July, has actively conducted public activities by meeting with the press and attending a forum as a panelist since late December.

A government source said South Korea's spy agency has decided to put on hold Thae's public activities, including media interviews, on concerns that he could be a target of North Korean agents.

But Thae said that he will continue to conduct public activities to reveal the brutality of the North Korean regime.

"Even if I face threats, I will not stop activities that can bring forward inter-Korean unification," Thae said on the TV program.

In 1997, Lee Han-young, nephew of Song Hye-rim, was shot in front of his apartment outside Seoul, 15 years after he defected to the capitalist South. Song is the mother of Kim Jong-nam.

Investigators suspected that Lee was the target of an assassination by North Korea. (Yonhap)