Malaysia's health minister said the dose of nerve agent given to North Korean ruler Kim Jong Un's exiled half brother was so high that it killed him within 20 minutes.

Ri Jong Chol, a North Korean arrested in the probe into last week's murder of the half-brother of the isolated state's leader, lived in Malaysia for more than three years without working at the company registered on his employment permit or receiving a salary.

Malaysia's ambassador to Pyongyang says the investigation into the death of the exiled half-brother of North Korea's ruler is being conducted in an impartial manner.