"Kim Jong-nam told KLIA staff that his face was feeling extremely painful because of an unidentified liquid sprayed at him. He was then taken for treatment at KLIA (Kuala Lumpur international airport) Clinic," Abdul Samah Mat, a Malaysian police chief, told The Daily Telegraph.

The results of a post-mortem into the 46-year-old's death are expected on Wednesday. Police are also investigating airport CCTV footage to try and identify the assailants and have ordered a toxicology test on the liquid used in the attack.

"He was taken to the KLIA (Kuala Lumpur International Airport) clinic for further treatment, but because of the condition he was in, he was rushed to Putrajaya hospital, but passed away soon after arriving," Mohmad Salleh, a Malaysian police chief, told the Telegraph.

In Washington, an unnamed government source said the US believed that North Korean agents were responsible for the murder.

Mr Kim, the son of former North Korean leader Kim Jong-il, was was once tipped to suceed his father but publicly criticised his family's dynastic control of the isolated state.

He also fell out of favour when he was caught trying to enter Japan on a false passport in 2001, saying he wanted to visit Tokyo Disneyland.

It would not be the first time the North Korean dictator has killed one of his own relatives. In 2013, he ordered the execution of General Jang Song-thaek, his uncle and mentor.