On Friday, US warplanes attacked and destroyed three oil tankers that were traveling through Syrian Kurdish territory. At least four people were killed in the course of the attacks. 

Details are still scant, and the US hasn't made a formal statement on what happened. The tankers are believed to have entered from Iraq, and were heading toward Assad territory. Since Syria lost  all their oil-producing regions in the war, they have to import what minimal oil they can get to keep their economy and military going.

But the US has imposed an oil embargo on Syria, in keeping with long-standing goals of regime change there. It seems the US ban on Syria having any oil is the basis for US warplanes attacking oil-containing vehicles in Syria. 

While the US attacking things in Syria generally has little in the way of immediate consequences, the Kurdish YPG, a US ally, have been believed to be profiting off these oil shipments as well, and the US attack may strain relations with them.