Iran rejected a package of economic and technology incentives offered by the European Union to end its nuclear program.
"The proposals are unacceptable," top nuclear negotiator Hossein Moussavian said, describing them as a "clear violation" of agreements between Tehran and the EU.
"They negate Iran's inalienable right," he added.
EU officials have called for an emergency meeting of the International Atomic Energy Agency, (IAEA), which could send Iran's nuclear file to the UN Security Council for possible sanctions.
But Moussavian said Iran's decision to resume uranium enrichment at its Isfahan plant was "irrevocable" despite the threat of possible UN sanctions.
He also warned Thursday that if EU negotiators called an IAEA meeting, Iran may not keep its freeze on enrichment at the Natanz plant.
On the other hand, the European Union said Iran must commit "not to pursue fuel cycle activities" if it wants to benefit from the EU incentives.
The set of proposals, presented by ambassadors from Britain, France and Germany, covers nuclear, political and security issues, and offers trade and technological cooperation.
The EU said it reaffirmed "Iran's inalienable rights to the peaceful use of nuclear energy, exercised in conformity with the nuclear Non-Proliferation Treaty," but also wants it to abandon sensitive nuclear activities.
An EU letter "offered to supply nuclear reactor fuel as well as security, technology and trade guarantees in return for Iran dropping parts of its nuclear programme that could be used to build atomic bombs."
"As Iran will have an assured supply of fuel over the coming years, it will be able to provide the confidence needed by making a binding commitment not to pursue fuel cycle activities other than the construction and operation of light water power and research reactors," the EU letter said, referring to less-sensitive nuclear activities.
It also stated that building a heavy-water reactor in the city of Arak "gives rise to proliferation concerns" and that Iran should terminate the project.
The three EU states are said to have offered recognition of Iran's right to produce nuclear power, as well improved trade ties with the EU, and guarantee of alternative nuclear fuel sources from Europe and Russia.
In exchange the EU wants Tehran to stop nuclear enrichment and construction of a heavy-water reactor.
The package was submitted after nine months of negotiations between the so-called EU "big trio" and Iran, which voluntarily agreed in November to freeze uranium enrichment.
The offer comes days after Iran's new President Mahmood Ahmadinejad took office, raising international concerns about the future of the country's foreign and nuclear policies.
The United States accuses Iran of covertly developing an atomic weapons project. But Iran denies the U.S. claims and insists that its nuclear plans are strictly for the peaceful generation of electricity and that it has a right to enrich uranium under the NPT.
Earlier, French Foreign Minister Philippe Douste-Blazy said the world will face a "major international crisis" if Iran rejects the proposals. "I hope Iran will heed the voice of reason."
But if the Islamic republic decides to resume enrichment, "then it is certain that the international community will ask the Security Council to intervene," he said.