A senior Iranian official blamed the U.S. Sunday for Tehran's refusal to respond to accusations it tried to make nuclear weapons, saying information provided by Washington was not only fake but came three years too late.

Ali Ashgar Soltanieh, Iran's chief delegate to the International Atomic Energy Agency, also acknowledged that his country's uranium enrichment program -- under sanctions by the U.N. Security Council -- was experiencing "ups and downs." The comment appeared to be the first instance of Tehran acknowledging that its enrichment activities were running into some difficulties.

The United States rejected the notion that it was at fault. Gregory L. Schulte, Soltanieh's American counterpart, said "Iran did not need to wait for information to answer" the accusations coming at it from all sides that it was trying to make nuclear arms.

"Iranian authorities could have started explaining these activities years ago, if only they had made the decision to come fully clean about their program."

Schulte and Soltanieh spoke separately to The Associated Press in the wake of an IAEA report saying that suspicions about most past Iranian nuclear activities had eased or been laid to rest.

But the report also noted that Iran had rejected documents that link it to missile and explosives experiments and other work connected to a possible nuclear weapons program, calling the information false and irrelevant.

Calling weaponization "the one major ... unsolved issue relevant to the nature of Iran's nuclear program," the report also confirmed that Iran is defying U.N. Security Council demands that it suspend enrichment, which can generate nuclear fuel and the fissile core of warheads.

Most of the material shown to Iran by the IAEA on alleged attempts to make nuclear arms came from the U.S. with some of its allies providing lesser amounts, diplomats told the AP, and the agency shared it with Tehran only after the originating nations gave their permission.

But Soltanieh dismissed much of the material as counterfeit. In any case, he said, it came too late -- three years after U.S. intelligence claimed it had material on a laptop computer smuggled out of Iran indicating that Tehran had been working on details of nuclear weapons, including missile trajectories and ideal altitudes for exploding warheads.

"They should have given it to us three years ago," Soltanieh said of the U.S. material, suggesting Tehran would then have had a more substantive response. Instead, he said, Iran did not get an offer for a review until mid-February. By that time, he said, the deadline for the conclusion of the IAEA probe into Iran's nuclear past had passed and experts were already working on the agency's report.

"All of a sudden, the Americans notice this thing is going to be closed," he said, referring to the probe. "So ... suddenly ... they have additional and new documents -- these dirty games should be stopped immediately."

Soltanieh acknowledged that Iranian experts also were offered some U.S. documents earlier than mid-February. But, he said, "We weren't allowed to take them out of the room," dooming any serious attempt to examine them.

"Some of the drawings were lousy and without any technical justification," he said, dismissing the material as "fabricated and (a) forgery."

On Saturday, Iranian President Mahmoud Ahmadinejad called on the U.S. and its allies to "apologize" for accusing Iran of seeking nuclear weapons. He asserted that the IAEA report vindicated Iran and warned that Tehran would take unspecified "decisive reciprocal measures" against any country that imposed additional sanctions against his nation.

Ahmadinejad and other Iranian leaders have depicted Tehran's enrichment program as a triumph and dismissed suggestions of technical glitches coming from U.S. intelligence, IAEA officials and independent experts.

Soltanieh on Sunday also asserted that Iran was "a master of enrichment technology." Still, he acknowledged that "during the process of development, there will be ups and downs and trials and errors."

"The important thing is ultimate success," he said.

Indirectly suggesting the existence of technical problems, Friday's IAEA report noted that Iran had not expanded its main enrichment project for months, keeping its number of centrifuges, which turn out enriched uranium, at 3,000. And former U.N. nuclear inspector David Albright, in his own report mailed to the AP, said the small amounts of enriched uranium produced over the past year "indicate that the centrifuges continue to operate below capacity."

"It is a surprisingly low output," Albright, whose Washington-based Institute for Science and International Security tracks countries under nuclear suspicion, separately told the AP.

The IAEA report will be the focus of discussions at an IAEA board meeting starting March 3. At that meeting, the U.S. and its allies will weigh whether to ask the board to approve a resolution declaring that the agency was unable to shed light on Iran's suspected nuclear weapons program, according to diplomats.