Washington is pushing the Islamic republic to state clearly by next week whether it will accept the offer -- a package of economic, trade and political incentives -- and suspend its controversial uranium enrichment activities.

Some countries fear Iran is trying to covertly develop nuclear weapons under the guise of a civilian atomic programme, but the government in Tehran maintains that it only wants to generate electricity.

Amid the rhetoric, Solana's talks with Ali Larijani -- their first face-to-face since the offer was made on June 6 -- are expected to provide a first real indication about Iran's intentions.

"We want tomorrow's meeting to be one that allows us to advance toward negotiations with Iran," Solana's spokeswoman, Cristina Gallach, said Tuesday.

The offer -- by Britain, France and Germany (the so-called EU-3) as well as China, Russia and the United States -- was received with optimism but senior Iranian officials have since suggested that it might be turned down.

For Solana though, Larijani's reaction is the one that counts.

"It's important to know officially what they think after all the various public declarations," Gallach said. "We will see if it's a first response, a last response, final or temporary."

In London, British Prime Minister Tony Blair suggested the meeting would be a good time for Iran to say its piece.

"I would like a response as soon as possible because I don't really see what more there is to talk about," he said, and added: "I don't set a deadline, but it would be interesting if there was an indication given at tomorrow's meeting for example of where the Iranians really stood on this question."

Early signs are that Iran will reject any imposed deadline.

Larijani was quoted Tuesday as saying that early August might be possible.

"Our negotiations with the Europeans will be on Wednesday, but it is only the beginning of the talks and our definite response to their proposals will be ready around the middle of (the Iranian month of) Mordad," or around August 6, he said, according to state-run television.

Iranian President Mahmoud Ahmadinejad had set August 22 as the date.

But US President George W. Bush, who refuses to rule out military action, wants a quicker answer.

"It seems like an awful long time for a reasonable proposal," he said late last month. "It shouldn't take the Iranians that long to analyse what is a reasonable deal."

Russian President Vladimir Putin wants to start nuclear negotiations with Iran before next week's Group of Eight summit in Saint Petersburg.

Under the offer, the six powers affirm Iran's right to develop nuclear energy, support its building of light water reactors and provide for enrichment to take place in Russia.

It would see Tehran's access to international markets and capital improved and give backing for it to join the World Trade Organisation, among other incentives.

In return, Iran would suspend all enrichment-related activities and accept wider inspections by the International Atomic Energy Agency (IAEA), according to a text shown to AFP.

The sticking point is that the Islamic republic considers enrichment to be a non-negotiable "red line." Indeed, Larijani said Monday that the condition of suspension "is not a reasonable proposal".

The talks in Brussels come after the IAEA reported Iran to the UN Security Council in February for hiding its sensitive nuclear work and losing the confidence of the West by breaking a suspension of enrichment activities.

Should they fail, Europe and the United States are likely to return to the Security Council to try to convince permanent members Russia and China to strengthen the IAEA's capacity to deal with Iran.

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