Foreign Ministry spokesman Hamid Reza Asefi also said Iran respond to a European proposal in the nuclear standoff with Iran once it receives it officially - but he underlined that any deal must respect Iran's right to enrich uranium.
The Europeans are considering a proposal to back off U.N. Security Council pressure on Iran if Tehran agrees to return to a freeze of its enrichment program. But the package would also include a stick, saying Iran could face sanctions backed by the threat of force if it refuses to suspend the program.
"With another deadline approaching, when U.S. and European Union nations will finalize a package to present to Iran, there has been no headway in getting the other Security Council members, including Russia and China, to agree to the sanctions being considered if Iran does not comply," said CBS News Foreign Affairs Analyst Pamela Falk. "There is some hope, however, that the U.N. nuclear chief, Mohamed ElBaradei, will be able to find common ground when he meets with U.S. negotiators in Washington before the package is presented," added Falk.
The top five powers at the Security Council, plus Germany, are still working on the draft and are to approve a final version on Wednesday.
"Any judgment about the [European] package is not mature. Nothing has been officially presented to Iran yet. One should not be hasty," Asefi told reporters.
"However, any package has to guarantee Iran's rights. We are waiting to receive the European package and will react to it subsequently," he said. "The basis of our work is clear. We won't get back to the past. We won't stop uranium enrichment."
Enrichment, in which uranium gas is spun in centrifuges, can produce either fuel for a reactor or material for a nuclear warhead. The United States accuses Iran of seeking to produce weapons, though Iran insists it intends only to generate electricity.
Iran heightened international concerns by announcing April 11 that it had enriched uranium for the first time with 164 centrifuges. Diplomats in Vienna, where the U.N. nuclear watchdog is based, later said it appeared Iran used Chinese uranium gas it obtained in the 1990s to feed into centrifuges because its domestically produced uranium gas was too impure.
"It is not correct. We don't use any foreign materials for enrichment. Uranium enriched at Natanz is from the uranium gas produced at Isfahan Uranium Conversion Facility," Asefi told reporters.
"The gas produced in Isfahan is of high quality," he said.
Iran has informed the International Atomic Energy Agency that it plans to install 3,000 centrifuges in the last quarter of 2006. Iran's final aim is to install 54,000 centrifuges at Natanz to annually produce 30 tons of nuclear fuel, enough to run a 1000-megawatt nuclear power plant.
The Isfahan facility converts raw uranium into hexaflouride gas, the feedstock for enrichment.
Technically, many impurities remain in the gas produced from processed uranium ore, or yellowcake. The gas conversion facility was built on a Chinese design, but Beijing backed out of the project under U.S. pressures in 1998, leaving the Iranians without Chinese expertise to ensure the best product.
Once uranium gas is produced in Isfahan, it is taken to the Uranium Enrichment Plant in Natanz, central Iran.
The gas is pumped into a centrifuge, which spins, causing a small portion of the heavier, more prevalent uranium-238 isotope to drop away. The gas then proceeds to other centrifuges - thousands of them - where the process is repeated, increasing the proportion of uranium-235.
Enrichment typically starts out with a gas that is 0.7 percent uranium-235.
On Wednesday, President Mahmoud Ahmadinejad rejected a European plan to offer Tehran incentives, including a light-water nuclear reactor, in return for giving up uranium enrichment, saying it would be like trading "walnuts and chocolates for gold."