Any such deal would violate United Nations sanctions imposed on Iran in 2006 over its nuclear programme. Uranium ore, also known as "yellow cake", can be enriched to use for fuel either for reactors or nuclear weapons.
The report by the Associated Press news agency cited an intelligence report by an unnamed member nation of the United Nations' International Atomic Energy Agency.
It said Iran was willing to pay $450 million for the uranium and that the clandestine deal, to be sealed with Kazakh state employees acting without the approval of the Kazakh government, could be completed within weeks.
"Such fabrications of news are part of the psychological warfare (against Iran) to serve the political interests of the hegemonic powers," Iran's representative at the United Nations said in a statement faxed to Reuters.
The United States and its European allies accuse Iran of planning to produce a nuclear bomb and are weighing further sanctions. Tehran says it only wants to generate electricity.
A U.S. thinktank said earlier this year Iran may be close to exhausting its yellow cake supplies and lacks the resources to sustain processing and enrichment activities.
A Western diplomat in Vienna said his country was aware of the intelligence and was concerned about it, but could not say whether or not it was accurate.
Kazakhstan denied the report.
"Everything Kazakhstan does in the uranium industry is done in accordance with the IAEA standards and under the control of this organisation," Kazakhstan's Foreign Ministry spokesman Yerzhan Ashykbayev said.
The amount of ore mentioned in the report would account for almost a tenth of Kazakhstan's 2009 output. The spot market price for 1,350 tons of yellow cake would be around $130 million, less than a third of Iran's reported bid.
Kazakhstan's state nuclear company Kazatomprom said on Wednesday it would produce 13,900 tonnes of uranium this year and 18,000 in 2010.
More than three-quarters of the total is produced by joint ventures with companies such as France's Areva and Canada's Uranium One.
The IAEA declined to comment on the report. (Reporting by Ramin Mostafavi; additional reporting by Boris Groendahl and Eric Onstad in Vienna and Olzhas Auyezhov in Almaty; editing by Philippa Fletcher)
