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

Some Western countries fear that 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 heated rhetoric, Solana's talks with Ali Larijani -- their first face-to-face since Europe's top diplomat submitted the offer 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 -- made by Britain, France and Germany (the so-called EU-3) as well as China, Russia and the United States -- was first 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 is important to know officially what they think after all the various public declarations," Gallach said.

"We need a first official response from Iran, so that this response can allow us to move forward," she said. "We will see if it's a first response, a last response, final or temporary."