"We are entering into a second phase which is the challenge posed by enrichment," Seyed Hossein Mussavian, a member of Iran's Supreme National Security Council, told the student news agency ISNA.
He said this new phase in ties with the International Atomic Energy Agency (IAEA) was "more difficult" because "the Americans and the Europeans are on the same side".
"The Europeans are saying that in order to be sure that nuclear fuel is not used to produce nuclear weapons, Iran must renounce enrichment.
"But Iran considers enrichment to be an absolute right in the Non-Proliferation Treaty (NPT) and Iran is not ready to renounce this," he said.
Tehran agreed to suspend enrichment under pressure from the IAEA, which is still probing suspicions that Iran is using a plan to generate atomic power as a cover for weapons development.
However, the country says the suspension is only temporary, and has continued to work hard on mastering other key parts of the sensitive nuclear fuel cycle.
European diplomats involved in the negotiations with Iran to cooperate with the IAEA have said they would prefer to see the Islamic republic obtain its nuclear fuel abroad and abandon fuel cycle work.
The question of enrichment will again be at the centre of discussion when the IAEA meets this week in Vienna, where Europe's big three -- Britain, France and Germany -- will present a draft resolution strongly critical of Iran's hiding of certain aspects of it programme.
Mussavian warned the IAEA's executive against adopting the text.
"If this resolution is adopted as it stands, that would signify that the Europeans, the IAEA and the Americans have a tacit agreement to keep the dossier at the top of the agenda so that the suspension of enrichment is longer," he said.
Iran, which flatly denies any interest in nuclear weapons, insists its case should be removed from the IAEA's top list of concerns.