It's a bit tricky to find the answer, since all the IRS documents are written to assume that the person reading them is subject to US tax law. What I have found is that "wages" are subject to Medicare and Social Security Tax, but that "Compensation paid to a nonresident alien (other than a resident of Puerto Rico) for services performed outside the United States is not considered wages and is not subject to withholding."
So your pay, at least when you are outside the US, is not "wages" under US tax law and is not subject to any withholding.
