State Department spokesman Heather Nauert warned that the killing of Kim Jong Nam made it clear that the “reckless” regime cannot be allowed to pursue a weapons programme.
The sanctions were announced hours after President Donald Trump had said “possible progress” was being made in talks with North Korea, but added that it “may be false hope”.
An increase in diplomatic efforts with North Korea has seen a delegation attend the Winter Olympics in Pyeongchang in February and on Tuesday Mr Kim met an envoy from Seoul.
South Korea said North Korea had offered to put its nuclear ambitions on hold, with a possibility of giving up its arsenal entirely, if the US agrees to enter a negotiation.
On Wednesday the UN said Secretary-General Antonio Guterres was “encouraged” by the developments and “stresses the need to protect the momentum and seize the opportunities available to find a peaceful path forward”.
However Spokesman Nauert’s statement illustrated the US’s desire to keep applying pressure on Pyongyang over its pursuit of weapons on mass destruction (WMD).
She said: “On February 22, 2018 the United States determined under the Chemical and Biological Weapons Control and Warfare Elimination Act of 1991 that the Government of North Korea used the chemical warfare agent VX to assassinate Kim Jong Nam in the Kuala Lumpur Airport.
“The United States strongly condemns the use of chemical weapons to conduct an assassination.
“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 (weapons of mass destruction) programme of any kind.”
The latest sanctions against the shadowy regime took effect on March 5, following the US’s finding in February that Pyongyang was behind the assassination.
The 45-year-old died after two suspects rubbed the VX nerve agent in his face as he waited for a flight home to Macau, where he lived with his family, on February 13, 2017.
Two women are on currently on trial for his murder in Malaysia.