Ukraine’s U.N. Ambassador Sergiy Kyslytsya responded: “None of the countries that provide assistance to Ukraine is under Security Council sanctions.” “Receiving assistance from the fully-sanctioned North Korea is a brazen violation of the U.N. Charter,” he added. “Sending the DPRK troops to support Russia’s war of aggression against Ukraine is a flagrant violation of international law.”
While it has been public since late '22 that special forces from a few NATO countries are in Ukraine for advisory and training purposes, I am not aware of any credible evidence for them participating in front line fighting (unofficial/volunteers don’t count).