Foreign Ministry spokesman Hamid Reza Asefi said the U.S. insistence on placing conditions on negotiations over a Western package of incentives has narrowed the scope of possible talks and made it tougher for all parties to reach a solution.
The incentives are meant to persuade Iran to stop enriching uranium, a process that can make nuclear fuel for a power plant or fissile material for an atomic bomb.
"We feel that the Americans are trying to take Europe to a point that the case could not be easily solvable," Asefi said. "The U.S. said it gave a deadline to Iran to respond to the package, but that is not correct. Again, they mix different issues and that is not appropriate."
Asefi reiterated that enriching uranium was his country's unalienable right, and that talks must be unconditional. He said Iranian officials were reviewing the package, and Iran would propose amendments to the deal.
"We have formed different committees to review the package. When the committees have concluded, we will send our answer to the Europeans immediately," Asefi said.
Iran has called the package a "step forward," saying some of the incentives were acceptable and calling for changes in others. It also said that the central issue of uranium enrichment needed clarification.
Iran denies accusations by the United States and others that it is seeking to develop nuclear weapons, saying its program would only generate electricity.
European Union foreign policy chief Javier Solana presented the package of perks and possible penalties to Tehran on June 6. The package was drawn up by the five permanent members of the U.N. Security Council — the United States, Britain, China, France and Russia — and Germany.
Crucially, the package calls on Iran to suspend, not permanently halt, uranium enrichment. Iran so far has said it will not give up its right under the Nuclear Nonproliferation Treaty to enrich uranium and produce nuclear fuel, though Tehran has indicated that it may temporarily suspend uranium enrichment to ease tensions.
The package included some significant concessions by the United States, including providing Iran with peaceful nuclear technology, lifting some sanctions and joining direct negotiations with Tehran.
However, it also contains the implicit threat of U.N. sanctions if Iran remains defiant.