As critics of the West's sanctions policy on Syria are noting: the European Union has for years allowed advanced weaponry to flow into the hands of anti-Assad jihadists, but it will act swiftly to block vital oil access to the war-torn and starved population. 

According to Reuters:

The Grace 1 tanker was impounded in the British territory at the mouth of the Mediterranean Sea, after sailing around Africa from the Gulf. Shipping data reviewed by Reuters suggests it had been loaded with Iranian oil off the coast of Iran, although its documents say the oil is from neighboring Iraq.

Reports say Gibraltar authorities (Gibraltar is a British Overseas Territory) acted on EU sanctions that have been in place for years against Syria; however one EU sanctions and legal expert told Reuters: "This is the first time that the EU has done something so public and so aggressive. I imagine it was also coordinated in some manner with the U.S. given that NATO member forces have been involved."