Interpol has issued a red notice on four North Korean suspects wanted in connection with the murder of Kim Jong-un's estranged half-brother, Kim Jong-nam, Malaysia's police chief has said.

Jong-nam, 45, was killed on February 13 when he was attacked with the deadly toxin VX nerve agent inside Kuala Lumpur International Airport.

Two women have already been charged with the murder and last month police requested Interpol's help to apprehend a further seven North Korean suspects who they believe fled to Malaysia shortly after the murder.

The four who are subject to the red notice - which is similar to an international arrest warrant - are believed to have made their way back to North Korea's capital Pyongyang.

Jong-nam, who is believed to have at least six children, had been living in exile for several years in the Chinese territory of Macau.

He had openly criticised his family's dynastic control of North Korea.

North Korea has questioned the Malaysian investigation into the murder and refused to acknowledge that the murdered man is Jong-nam.