WASHINGTON (REUTERS) - The United States has decided Pyongyang used the chemical warfare agent VX to assassinate the half-brother of North Korean leader Kim Jong Un in Malaysia in 2017 and has imposed additional sanctions on the country, the US State Department said on Tuesday (March 6).

State Department spokeswoman Heather Nauert said in a statement the US government made the determination on Feb 22 under the Chemical and Biological Weapons Control and Warfare Elimination Act of 1991.

"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," she added.

Additional sanctions on Pyongyang went into effect on March 5 after the finding was formally published, she said.

Mr Kim's estranged half-brother, Kim Jong Nam, was killed at the airport in Kuala Lumpur when two women smeared his face with the banned chemical weapons agent VX.

The two women, Indonesian Siti Aisyah and Vietnamese Doan Thi Huong, have been charged with murdering Mr Kim and the trial is ongoing in Malaysia.

The US State Department did not explain how it reached the conclusion that North Korea used chemical weapons on its own nationals. Neither did it give any names of any other victims apart from Kim Jong Nam, whose assassination last year garnered massive media attention.