"We are definitely not going to compromise on our rights. We are going to act according to our responsibilities and rights. We are holding the initiative," foreign ministry spokesman Hamid Reza Asefi said.
He was repeatedly asked whether Iran would return to a suspension of enrichment, but only said that "we will not negotiate our right. We will not give up our right."
Mr Asefi declined to categorically say if Iran would reject or accept an international offer of incentives if it agrees to stop enrichment -- a process at the centre of fears the country could acquire nuclear weapons.
According to Mr Asefi, the package contained elements which were "acceptable" and others which were not. The package was drawn up Britain, France and Germany and backed by the United States, Russia and China.
"As soon as we received it, we opened it and started to study it. We categorised the points. Some of them are acceptable, some have ambiguities, some have to be consolidated, and some of them we don't accept," Mr Asefi said.
Iran's top nuclear negotiator, meanwhile, on a visit to Cairo, said that Tehran would not accept any threats or preconditions in negotiations with the international community.
"The language of threats contradicts the language of negotiations," said Ali Larijani.
US President George W Bush said on Friday that Tehran had "weeks and not months" to accept the proposal to halt its nuclear program and warned that the UN Security Council would act if Iran did not comply.
But Mr Larijani denied that any deadline had been fixed in the proposal and rejected any preconditions. "There is no timeframe ... We are for negotiations but without preconditions," he said.
Iran has so far refused to suspend enrichment, which can be extended from making civilian reactor fuel to the core of a nuclear weapon.
The country argues enrichment for peaceful purposes is a "right" enshrined by the nuclear Non-Proliferation Treaty.