The free credit for 180 days -- offered to "a handful" of potential customers in Asia, including India -- amounts to a 7.5 percent discount per $118 barrel, the officials told the Financial Times.
Each month of credit amounts to a discount of roughly $1.20 to $1.50 a barrel, they estimated.
But even with zero-percent interest -- made popular in the United States by the auto industry to lure customers into empty showrooms during the 2008-2010 crisis -- Tehran is struggling to find customers, Gulf-based officials and European traders told the British business newspaper.
"Obviously, [the extra credit is] the easiest way for them to discount," a senior European oil trader said. "However, I think very few will be tempted."
Saudi Arabia and other leading Middle East oil producers typically extend 30 days of credit. Tehran in the past let importers such as China pay in 60 to 90 days, the newspaper said.
The report came ahead of talks, to begin Friday, between Iran and the United States, Britain, China, France, Russia and Germany on Iran's nuclear program -- talks that broke off in stalemate more than a year ago.
U.S. and European officials, supported by U.N. weapons inspectors, maintain Iran plans to build nuclear weapons. Iran's leadership insists the government's goal is for peaceful civilian uses only.
Saeed Jalili, Tehran's chief nuclear negotiator and Supreme National Security Council secretary, predicted Wednesday the nuclear talks would succeed only if the six world powers arrived with "a constructive approach" and refrained from coercive tactics, Iran's state-owned al-Alam news channel reported.
"The language of threat and pressure has never yielded results and only reinforces the determination of the Iranian people," it quoted Jalili as saying.
The six powers had no immediate response.
The report of zero-percent interest also came two days after Iranian Petroleum Minister Rostam Ghasemi insisted Iran would have no problem selling its oil, despite the economic sanctions and an impending European embargo of Iranian oil, to start July 1.
"Iranian oil has high economic value, and the international oil market would never neglect it," the official Islamic Republic News Agency quoted him as saying.
But even before the oil embargo goes into effect, European purchasers of Iranian oil have started buying elsewhere, The New York Times reported.
In retaliation, Tehran suspended oil exports to Germany and planned to stop shipments to Italy, al-Alam and state-run Press TV reported.
It halted crude exports to Spain Tuesday and cut off oil sales to France and Britain in February, the media said.
The International Energy Agency oil watchdog said Iran's crude production had now reached a 10-year low.
Iranian President Mahmoud Ahmadinejad said Tuesday Iran had so much hard currency in reserve it would "manage well, even if we don't sell a single barrel of oil for two or three years."