To me, it means "against a law", and laws are made by countries (sure, parliaments of those countries or dictators or...), and generally apply only to that particular country (some things attempt to get a wider reach, but they are usually unenforceable unless there's a local company to pursue, most famous example being GDPR).

There are international conventions and the UN, but countries do not have to be signatories or members to any of them. And I've never heard anyone use the term "illegal" in that sense before.

So what do you mean with "clearly illegal"?

(fwiw, I am very much against the US acting as the "policeman of the world", but sanctions are a political tool to make someone less powerful comply; beats an invasion and bombing that USA has frequently resorted to)