World powers are ready to take up Iran's call for talks over its nuclear aims but only if it first suspends uranium enrichment, France said yesterday with a UN deadline for Tehran to halt the programme approaching.

Iran handed over its answer to an incentives offer from six nations on Tuesday, saying it contained ideas that would allow serious negotiations to start immediately. Previous such calls for dialogue have been seen by the West as a stalling tactic.

"As we have always said ... a return to the negotiating table is tied to the suspension of uranium enrichment," French Foreign Minister Philippe Douste-Blazy told a news conference.

Diplomats said Iran's response did not accept this precondition, which it previously dismissed as worthless.

Also yesterday, the semi-official Mehr news agency said Iranian authorities would announce a "very important achievement" in an area of nuclear technology within days. Mehr quoted an unnamed official and did not specify what the achievement was.

The UN Security Council has warned Iran could face sanctions if it does not meet an August 31 deadline to freeze enrichment work, a process that can be used to make fuel for nuclear power plants or material for warheads.

The five permanent Council members Britain, China, France, Russia and the US, as well as Germany offered Iran economic and other incentives to stop enrichment. So far, they have not given their verdict on Iran's reply.

Two European diplomats said Iran's 21-page reply ruled out shelving enrichment activity before any talks but indicated that it might be open to doing so in the course of negotiations.

"It doesn't rule out a suspension, but it asks that talks start first, before a suspension," said one diplomat.

"It is not acceptable at first look. But we now must make a judgement whether a change in tactics is merited.

"It is not a rambling letter, like the earlier letter from President Ahmadinejad (to US President George W. Bush)," the diplomat added. "It is also not as tendentious. It deserves a considered response."

Analysts say Iran's answer, described by diplomats as complex and nuanced, was probably designed to divide Security Council members Russia and China, both key trade partners of Tehran, from the US, Britain and France, which have backed tougher sanctions. All five have a veto on the Council.

"In the reply, although (the West) has taken destructive measures, Iran has tried to create new opportunities for resolving the issue," senior Iranian nuclear official Ali Hosseinitash told Iran's official news agency IRNA.

Mr Douste-Blazy described the reply as "a very long, complex document" and said the six powers would decide in a few days what to do in the Security Council. One of the European diplomats said the sextet would reserve judgement pending a report from UN nuclear watchdog chief Mohamed ElBaradei, due today week, that will certify whether Iran has stopped enrichment-related work or not.

EU foreign policy chief Javier Solana, who delivered the offer to Iran two-and- a-half months ago, said the reply "requires a detailed and careful analysis".

The White House said on Tuesday that Mr Bush had yet to examine the reply. The US ambassador to the UN, John Bolton, said Washington was set to move quickly on a resolution seeking sanctions if Tehran rejected the incentives offer.

A Chinese Foreign Ministry statement urged Iran to consider international concerns and take "constructive steps... We also hope that other parties remain patient and calm," it said.

Russian Foreign Ministry spokesman Mikhail Kamynin, reflecting Moscow's reluctance to take punitive action against Iran, was quoted by Interfax news agency as saying: "Russia will continue with the idea of seeking a political, negotiated settlement concerning Iran's nuclear programme."

The world's fourth-largest oil exporter, Iran says it needs to enrich uranium as a peaceful, alternative energy source and has a right to do so under the nuclear Non-Proliferation Treaty.