The United States formally concluded on Tuesday that North Korea ordered the murder of a half-brother and potential rival to ruler Kim Jong-un with the VX nerve agent.

The North Korean leader's estranged half-brother, Kim Jong-nam, was killed at the airport in Kuala Lumpur in February last year when two women smeared his face with the banned chemical weapons agent VX.

Heather Nauert, the State Department spokeswoman, said in a statement the US government made the determination on February 22 under the Chemical and Biological Weapons Control and Warfare Elimination Act of 1991. Additional sanctions on Pyongyang went into effect on March 5 after the finding was formally published, she said.

"This public display of contempt for universal norms against chemical weapons use further demonstrates the reckless nature of North Korea and underscores that we cannot afford to tolerate a North Korean WMD programme of any kind," Ms Nauert said on Tuesday.

The fresh layer of US economic sanctions against Pyongyang came just as South Korea reported that the regime is ready for talks to end a nuclear standoff.