SK64 pilot reported a lack of flight time limitations of U.S. registered aircraft operations in foreign countries resulted in severe fatigue situations.

Narrative

Under part 91 there are no Flight Time and Duty Limitations and there should be!I advocate for many pilots like myself who are sometimes too tired but need to shut up to keep their employment. Many incidents due to tired crew could have become an accident!In the US the matter is regulated by an organization with a set of reasonable limitations. In other Foreign Countries; there are more stringent sets of FTL (Flight Time Limitations) which regulates the matter. Unfortunately when we operate an 'N' registered aircraft in firefighting operation under Part 91 abroad; the Operator tell us that there are no FAA limitations under Part 91 and we can work 16 hours or more with more than 10 hours of flight a day on fires; in 40°C ++ OAT which translates to 50°C in the cockpit with no AirCondition units.This is something that should be looking into and at least try to match the FTL required by other organizations.A better idea would be to consult other Aviation Authorities and try to find a good international common ground; so that crews operating 'N' registered aircraft; when abroad in Unregulated Countries; will have a minimum of safety frame within which to set up the schedule.I have been in rotations where; for continuous 21 days shift; I had no REST DAYS; duty days in excess of 14-15-16 hours leaving definitely few hours of sleep (4-5 hrs) too little to recover from sleep deprivation. The chief pilot said that speaking with the organization; we do not need to adopt more stringent local rules; unless they are requested in the contract; and that 91.703 of the regulation is limited to the maneuvering of the aircraft. Not FTL.There are few companies operating big helicopters abroad in this specific operations and I understand that we are not many pilots involved; but one single life saved would be worthed. If the majority of other international Agency/Authorities has taken the matter under scrutiny it is because of accidents in the past leading to overtired fatigued crews. It would be wise if we were in the lead and not the last to come up with some regulations in this field of aerial application; similarly to part 135 or similar to the Part 91 Subpart K for the fractional ownership.Maybe call it Part 91. Subpart FF (for firefighting activities).