Ahmadinejad waved away a copy of the purported document when presented it by ABC news anchor Diane Sawyer during a sit-down interview, according to excerpts released ahead of its scheduled airing Monday.
"No, I don't want to see this kind of document. These are some fabricated papers issued by the American government," he says through a translator, in his first public remarks on the matter.
Tehran's foreign ministry spokesman has previously dismissed the claim as a "scenario" hatched by Western powers.
The British newspaper the Times reported last week that foreign intelligence agencies dated the documents to early 2007 — four years after US agencies had assessed Iran had suspended efforts to produce nuclear weapons.
The US State Department said Tuesday it would investigate the report, adding the "revelation" fueled concerns about Iranian intentions.
It said the documents detailed a plan to test whether the device works — without leaving traces of uranium that the outside world could detect.
Asked on ABC's "This Week" program on Sunday about Ahmadinejad's assertion that the documents were a US fabrication, senior White House David Axelrod dismissed the talk as "nonsense."
"Of course that's nonsense. Listen, nobody has any illusions about what the intent of the Iranian government is," Axelrod said.
"And we've given them an opportunity to prove otherwise by allowing them to ship their nuclear material out to be reprocessed for peaceful use. And they have passed on that deal so far. And the international community is going to have to deal with that if they don't change their minds."
Axelrod said Tehran would face "consequences" if it did not change direction, with time is running out ahead of a year-end deadline for Iran to seize the US offer of diplomatic engagement for resolving nuclear and other issues.
"When we came to office, Iran was united and the world was divided in an approach to deal with Iran. Today, Iran is bitterly divided, and the world community has come together, and the president has been a big force in bringing them together," Axelrod said.
"And I think that the world is united and is willing to take additional steps if the Iranians don't turn around," he said.
"Plainly, there are going to be consequences if they don't turn around."
Iran insists its nuclear program is solely for civilian purposes and rejects Western suspicions that it is covertly trying to develop a bomb.
The US-based Institute for International Science and Security (ISIS) has said the documents, which it said The Times showed its experts before publishing the report, could be a "smoking gun" showing proof of what the Iranians are up to.
"But ISIS urges caution and further assessment of this document, in particular to confirm the document's date and with how the document fits with other information regarding Iran's nuclear weaponization activities both prior to 2003 and any work afterwards," it said on its website.
