The United States has formally concluded that North Korea ordered the murder of ruler Kim Jong-un's half-brother and potential rival with VX nerve agent.

The conclusion was drawn by US investigators operating under the 1991 Chemical and Biological Weapons Control and Warfare Elimination Act.

The finding has triggered another layer of US economic sanctions against Pyongyang, just as South Korea reported that the regime is ready for talks to end a nuclear standoff.

Kim Jong-nam died in 2017 after the highly toxic nerve agent was rubbed on his face by two women at Kuala Lumpur International Airport in Malaysia.

The women, who claim they were told they were carrying out a prank for a TV show, are on trial for the murder of Kim Jong-nam.

He was late North Korean leader Kim Jong-il's oldest son.

He had once been seen as Kim Jong-il's natural heir, and some reports had suggested that China might have been grooming him to replace the younger Kim Jong-un in the event of a crisis.