Usb standard is 5V, most chargers with usb output should be 5V. The Amp is determined by the Voltage and (electrical)resistantce, check Ohm's law.
However the chargers are probably limited to a certain amperage(max amp output). All chargers I've seen have inscripted input and output values, as long as output values are within usb standards it should be ok.
I have an iphone car charger with usb out. It stays in the car and charges my family's gadgets no mather the manufacturer/type of device, as long as it supports usb charging.
So theoreticaly it should work. To be safe check the output values on the usb charger.
If n1 came with the american charger you can check the output values of that against the iphone charger output values.