VX is a chemical weapons agent listed as a weapon of mass destruction by the United Nations and its use is banned under the Chemical Weapons Convention. The finding corroborates suspicions that the North was behind the killing.

Eight North Koreans, including those holding a diplomat's or official passports, have also been named suspects in the case, but Pyongyang has denied involvement, accusing South Korea of colluding with Malaysia to incriminate the communist nation.

"The use of this VX against his half-brother -- certainly, all indicators point to North Korean responsibility for this -- it is another example of his use of these types of toxins to carry out his objectives," John Brennan, who served as CIA director until January, said on CBS' "Face the Nation."

Brennan also said that the use of VX shows the North's leader "continues to use lethal means to eliminate either opponents of his regime internally or those who he just disagrees with."

"There has been brutal repression inside of North Korea over the last number of years under Kim Jong-un's leadership. He's killed many, many individuals in some of the most brutal means possible," he said.

In an attempt to consolidate his grip on power since taking over as leader following the death of late leader Kim Jong-il in late 2011, the North's leader has executed a number of senior government and military officials, including the 2013 execution of his own uncle Jang Song-thaek. (Yonhap)