Apple is no longer letting customers buy an iPhone using cash - and has restored its previous limit on two iPhone sales per customer.

The company says the two moves are part of its attempt to stop unauthorised iPhone resellers. The company estimates 240,000 iPhones of the 1.4 million it had sold by the end of September have gone to individuals and companies engaged in unlocking the device.

Speaking to the Associated Press, Apple spokeswoman Natalie Kerris confirmed the no cash, two-only restriction came into effect on Thursday.

"Customer response to the iPhone has been off the charts, and limiting iPhone sales to two per customer helps us ensure that there are enough iPhones for people who are shopping for themselves or buying a gift," Kerris said. "We're requiring a credit or debit card for payment to discourage unauthorized resellers."