Dictionary definition of ethics is as follows:
a) A set of principles of right conduct.
b) A theory or a system of moral values.

So yes, ethics are based culturally. In order to impose a universal system of ethics you would really need to impose a world religion, political system or government and previous attempts to do this (Communism, etc) have failed. Had they succeeded, however, I imagine that they would have led to the death of ethics as a concept, due to a lack of any comparable system. Catch-22, methinks!

If the above doesn't seem to be particularly lucid, I apologise but a) I'm tired and b) political philosophy was never my strong point!