“We reached a preliminary agreement at the experts level,” a top negotiator, Hossein Moussavian, told state television by telephone from the French capital.

The agreement followed what has been described as two days of “difficult discussions”.

“This agreement is to be taken to the capitals of the four countries, and in the next days, if the capitals approve it, it will be announced officially,” he said, adding he was “not pessimistic”.

“Should the four countries be able to agree on a package in the not-so distant future, there would be a crucial transformation in relations between Iran and the EU and the whole international community.”

His announcement followed two days of talks between Iran and the EU troika of Britain, France and Germany.

The talks were aimed at getting Tehran to suspend uranium enrichment in order to avoid being hauled before the UN Security Council for possible sanctions.

The US accuses Iran of secretly trying to develop nuclear weapons under cover of its civilian atomic energy program.

Washington wants the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), to refer Tehran to the Security Council when the agency meets in Vienna on November 25.

But veto-wielding council members China and Russia have voiced their opposition to the issue moving out of the IAEA.

Russia is helping Iran build its first nuclear power plant in a deal worth about $A1.06 billion.

Mr Moussavian said the preliminary agreement included “a collection of political, economic, security, technological and trust-building cooperation regarding the future of Iran’s peaceful nuclear activities”.

But he refused to give any specific details pending its approval by “the most senior officials” in Iran as well as in the capitals of the EU’s so-called “big three”.

He also made no mention of whether Iran had agreed to meet EU demands that it halt its controversial work on the sensitive nuclear fuel cycle.

Officials in Washington insist those activities are aimed only at producing fuel for civilian reactors.

The Europeans fear the process surrounding the enrichment of uranium could eventually be used by Iran to build a nuclear bomb.

Tehran has until now resisted Europe’s demand for an indefinite suspension, arguing that it would infringe its right to maintain a civilian nuclear power program.

But a year ago Iran agreed to suspend enrichment, albeit while continuing other fuel cycle work, pending the completion of an IAEA probe.

Enrichment is permitted under the nuclear Non-Proliferation Treaty (NPT), the treaty overseen by the IAEA and to which Iran is a signatory for peaceful purposes.

However Iranian officials have said they were open to maintaining a “voluntary” suspension, but not a total halt, of enrichment, a step the IAEA board called for in September.