The govorning regulation is DOT CFR 49, Subpart D, 173.115(b).
Basically, if a tank is above 41psia, its a restricted HAZMAT, but if its below 41psia, its completely unregulated.
For international flight rules, the DOT referred me to International Civil Aviation Organization (ICAO) 1995-1996 edition, chapter 2 on page 2-2-1, which is pragmatically identical to the above. This does not guarentee that each country doesn't have their own rules, but is a general guide.
I have the complete text of DOT CFR 49, Subpart D, 173.115(b). online at URL: http://www.webspan.net/~hummer/flyingpony.html
This page also contains a DOT 800# to verify if its still current.
-hh ( H NED Huntzinger )