14 C.F.R. § 375.33
Foreign civil aircraft carrying passengers, property or mail for remuneration or hire, but not engaged in scheduled international air services, are authorized to navigate nonstop across the territory of the United States and to make stops for non-traffic purposes. The navigation of foreign civil aircraft in the United States is not authorized under this section when the elapsed time between landing and takeoff at a stop in the United States exceeds 24 hours and passengers are permitted to leave the airport or when passengers, property or mail are transferred to another aircraft. Flights involving stops under such circumstances may, however, be performed in the case of emergency relating to the safety of the aircraft, passengers, cargo or crew.
14 C.F.R. §375.33