Iran has repeatedly said it would end its freeze on uranium enrichment -- a key process in the nuclear fuel cycle -- if it is not satisfied with the progress of talks with the Europeans on its nuclear activities.

"One of our minimum (demands) is that the suspension is partially lifted," said a letter from Iran's chief negotiator Hassan Rowhani to the EU, negotiator Ali Agha Mohammadi told the student news agency ISNA.

The EU-3 -- Britain, France and Germany -- have been holding talks to encourage Iran to provide guarantees that its nuclear programme is not aimed at building the atomic bomb, in return for various trade and political incentives.

Tehran has previously simply sought EU recognition of its right to enrichment to be included in proposals due to be submitted by the Europeans by August 1.

"For negotiations to continue, the Europeans must accept the minimum reduction in the length of the suspension (called for) by Iran," said the letter.

Although Tehran accepted a temporary suspension of enrichment activities in November, the Islamic republic has consistently refused an indefinite halt.

Rowhani's letter said that "if European proposals don't take into account the Iranian request concerning enrichment, they will be considered null and void.

"If European proposals pose a problem, we will make it known to the people and the country will take the decision it wishes," it said, without elaborating.

"Then we may enter a difficult period."

More ominously, the man tipped to be Iran's next foreign minister, Ali Larijani, said "it is logical and just to expect the last act of this government to be the resumption" of enrichment-linked activities, the semi-official Fars news agency reported.

Iran has previously sought to reassure the Europeans that its nuclear policy would not change under ultra-conservative president-elect Mahmood Ahmadinejad who is due to take office early next month.

"With the change of government... the attitude of the Islamic republic with regard to the nuclear issue and our strategic objectives will not change and the next government will continue the same policy," a foreign ministry spokesman said on Saturday.

Iran insists that its nuclear programme is purely peaceful despite US claims it is seeking atomic weapons, and says it has the right to enrich uranium under the nuclear Non-Proliferation Treaty.

Enriched uranium is used to manufacture fuel for nuclear power stations but can also be used to manufacture atomic weapons.

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