Situation: should I disobey BdU orders?

[DirtyACE]

I'm currently on my first patrol in the Atlantic, about 200km north-west of Ireland. There's a convoy going to England, of which I sent a contact report to the BdU. They in return gave me an objective of targeting tankers within it. There are a few tankers, however, they're flying flags of nations (Greece and Panama) who are not yet in war with Germany. Should I go ahead and sink them anyway or disobey my superiors' orders?

One other question. I also noticed that the some of the Greek merchant ships have guns on them and they do try to shoot at me, even though they're supposed to neutral at this time. Can I sink these ships as well in that case, since they are being hostile to me?