A report by U.N. investigators that they have found no evidence of an Iranian nuclear weapons program "is simply impossible to believe," Undersecretary of State John R. Bolton (search) said Wednesday.

Bolton said Iran has enriched uranium (search) with both centrifuges and lasers and has produced and reprocessed plutonium (search).

"It attempts to cover its tracks by repeatedly and over many years neglecting to report its activities and in many instances providing false declarations to the IAEA," Bolton said in a speech at a dinner of The American Spectator magazine.

The U.N.'s International Atomic Energy Agency said in a report this week that Iran had been involved in numerous cases of covert nuclear activities, including uranium enrichment and the production of small amounts of plutonium that effectively put the nation in violation of the Nuclear Nonproliferation Treaty.

But it also praised Iran for cooperation and openness and said it had found no evidence of an Iranian nuclear weapons program.

Separately, a senior U.S. official said Iran has been able to confuse the IAEA with partial disclosures that will keep the agency from referring Iran's program to the U.N. Security Council this month for possible sanctions.

Iran's revelations to the IAEA show a nuclear capability far beyond civilian purposes, and Iran almost certainly could produce nuclear weapons by the end of the decade, the official said, speaking on condition of anonymity.

While maintaining it only wants to generate nuclear power, Iran has delivered what it says is complete information about past suspect activities to the IAEA.

Last month, Iran notified British, French and German officials it would suspend uranium enrichment and throw open its nuclear programs to unfettered agency inspections.

In Tehran, Iranian President Mohammad Khatami asserted Wednesday that the IAEA report dispelled suspicions Tehran was seeking atomic arms.

The British foreign secretary, Jack Straw, is expected to take up the issue in talks Thursday in Washington with Secretary of State Colin Powell.

"We should be reacting calmly to the latest report from the International Atomic Energy Agency," Straw told British Broadcasting Corp. radio.

Straw said that while Iran had concealed nuclear activities in the past, it had cooperated substantially recently with the IAEA.

Russia, meanwhile, has agreed to delay shipping fuel to Iran for the Russian-built nuclear reactor at Bushehr from next spring to next summer, the U.S. official said.