This story has been updated with comment from Sprint.

Sprint is offering new customers who buy any of its smartphones a $100 American Express gift card, its latest attempt to draw in postpaid subscribers.

The promotion runs through Aug. 26 and includes the iPhone 4S, which Sprint recently discounted to $149.

Including the gift card and discount, Sprint has effectively reduced the price of the iPhone 4S to $49 for new subscribers.

Customers must sign a new two-year contract with a data plan to qualify for the offer and have an account in good standing for 30 days before the card will be mailed. The promotion excludes upgrades for existing subscribers.

The fire sale comes ahead of Apple's rumored launch of a new iPhone next month, prompting speculation that Sprint is working to clear its inventory before the new smartphone hits shelves. A spokesman for Sprint said the promotion was "routine" and not linked to upcoming smartphone launches.

Sprint agreed last fall to pay Apple $15 billion over the next five years for the iPhone. CEO Dan Hesse has said the company does not expect to have difficulties fulfilling its purchase commitments. Sprint began selling a prepaid version of the iPhone through its Virgin Mobile USA brand in late June.

Sprint's rock-bottom smartphone prices could help it attract customers as it continues to look for ways to offset defections from its iDEN platform, set to be completely decommissioned by next summer.

Sprint lost 998,000 iDEN customers during the second quarter, declines offset by the addition of 1.28 million new postpaid, prepaid and wholesale customers on the Sprint brand. Its subscriber base stood at about 56.4 million at the end of June.