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 nobody has officially send troops, there are a significant amount of professional soldiers from NATO states in Ukraine. Some of them volunteered sure, but I would assume that there are also special forces there, for let’s say payed time off. This however does not change the original point, that the use of 10000+ NK soldiers changes the dynamic of the war.