In the FAR/AIM (rulebook) it says, and i quote:
61.123 (h) "Hold at least a private pilot certificate issued under this part or meet the requirements of 61.73"
61.73 is for military pilots
Where it says "issued under this part" it is refering to Part 61. So, what this is saying is that you must hold a US PPL in order to qualify - foreign PPL does not qualify. However, in part 61.75 it says that you can get a US PPL on the basis of a Foreign PPL and then probably qualify as stated by 61.123.
At least thats my intepretation of the FAR's.