It also lashes out at Britain and France for "ill will and political motivation" in their dealings on Iran.
Iranian envoy Ali Asghar Soltanieh sent the letter to Mohamed ElBaradei, the head of the International Atomic Energy Agency, whose 35-nation board will take a hard new look at Iran's nuclear program next week.
Iran insists its nuclear activities are peaceful and geared solely toward generating electricity. The U.S. and key allies contend the Islamic Republic is covertly trying to build an atomic bomb.
Tehran has bristled at the agency's latest report, which accuses Iran of defiantly continuing to enrich uranium and refusing to clear up lingering questions about possible military dimensions to its nuclear program.
In the letter, a copy of which was obtained by the AP, Soltanieh insists that Iran has demonstrated "the full commitment of my country to its obligations" under an IAEA nuclear safeguards agreement.
But it takes sharp aim at Washington for giving the U.N. nuclear watchdog unspecified intelligence and other evidence allegedly recovered from a laptop computer that reportedly was smuggled out of Iran.
U.S. intelligence later assessed the information as indicating that Tehran had been working on details of nuclear weapons, including missile trajectories and ideal altitudes for exploding warheads.
The material on the laptop also included videos of what intelligence officials believe were secret nuclear laboratories in Iran.
"By interfering in the work of the IAEA and exerting various political pressures, the government of the United States attempted to spoil the cooperative spirit between the Islamic Republic of Iran and the IAEA," the letter reads.
"The government of the United States has not handed over original documents to the agency since it does not in fact have any authenticated document and all it has are forged documents," Soltanieh said.
"The agency didn't deliver any original documents to Iran and none of the documents and materials that were shown to Iran have authenticity and all proved to be fabricated, baseless allegations and false attributions to Iran," he added.
"Therefore, this subject must be closed," Soltanieh wrote.
U.S. officials were not immediately available for comment late Friday. Officials at the French Foreign Ministry would not immediately comment.
A spokeswoman for Britain's Foreign Office denied the allegations in Soltanieh's letter.
She said Britain had consistently sought a way to "give diplomacy a chance to succeed."
"I would deny any suggestion of ill-will in the strongest possible terms," she said, speaking anonymously in line with department policy. "We would have no hesitation in saying that absolutely the reverse is true."
The IAEA itself has pressed the U.S. and other governments to share more details on Iran-related intelligence. In its latest report on Iran, the U.N. agency noted that "constraints placed by some member states on the availability of information to Iran are making it more difficult for the agency to conduct detailed discussions with Iran."
In a brief telephone interview Friday evening, Soltanieh told the AP he hoped the letter would pressure the U.S. to fully divulge the source of any intelligence implicating Iran.
"We are the victims of negligence, because people still don't know what this is all about," he said.
The nuclear agency's latest assessment did acknowledge that Iran has been producing nuclear fuel at a slower rate and has allowed U.N. inspectors broader access to its main nuclear complex in the southern city of Natanz and to a reactor in Arak.
But it cautions that there are "a number of outstanding issues which give rise to concerns and which need to be clarified to exclude the existence of possible military dimensions."
The report, to be examined next week, has raised the specter of harsher international sanctions against Iran for not answering lingering questions about its nuclear activities.
Senior U.N. officials have said Iran has been feeding uranium ore into some of its 8,300 centrifuges at a reduced rate, suggesting that sanctions already in place may be hampering its program.
As of Aug. 12, only about 4,600 of those centrifuges were actively enriching uranium, compared with about 4,900 in June - the last time the nuclear agency issued a report on Iran's nuclear activities - officials said. Since then, they said, Iran has installed roughly 1,000 more centrifuges, but it appeared that many were idle.
Soltanieh's letter contends the overall assessment on Iran is positive. But he says concerns raised by the U.S. and others have "totally overshadowed and undermined" the steps that Iran has taken to comply with IAEA demands for transparency.
President Barack Obama has given Iran something of an ultimatum: Stop enriching uranium - which, if done at a high level, can produce fissile material for the core of a nuclear weapon - or face harsher penalties. In exchange for stopping, it could get trade benefits from six countries that have been engaging it in separate talks: the U.S., Britain, China, France, Germany and Russia.
The U.N. Security Council has imposed sanctions on Iran three times since 2006 for its refusal to freeze uranium enrichment. The sanctions grew from fears that Iran is using the pretext of building a peaceful nuclear energy program to eventually make weapons-grade enriched uranium.
The country has also been placed on an international watch list to help limit the importation of nuclear materials, which could make it difficult to procure enough uranium oxide to feed its enrichment program.
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Associated Press Writer Raphael G. Satter in London contributed to this report.
