How do one country's sanctions on another apply to third parties?

Question

Having just read that Britain has seized a ship sailing under the flag of Panama carrying oil from Iran to Syria, I wonder what the legal basis for this is.

EU sanctions forbid oil exports to Syria. How can the EU enforce such sanctions on a vessel that has no ties to the European Union?

Edit: Apparently the vessel passed through European waters. That may make the difference.

Answer 1

The ship is tied to the EU

It was in EU territory (specifically the UK) and therefore subject to UK law.