boarded under international law
I think You might have misspelled "under US law".
Cause international law is pretty clear on when ships may be boarded in international waters, and US sanctions enforcement isn't on the list. Not that it makes any difference, because, as mentioned earlier, US is not party to UNCLOS. And maybe that's what they're going for - send a "clean", properly registered, tanker with "humanitarian" oil, have it seized by the US Navy and make a scene out of it in the UN.