Solana made the point in talks with Iranian Vice President Gholamreza Aghazadeh, visiting Brussels to discuss ties with the European Union long clouded by the nuclear issue.

"The main challenge of the discussions on the nuclear issue is to be able to reach these clear objective guarantees of the peaceful nature of the nuclear Iranian program," said Solana's spokeswoman Cristina Gallach.

Iran, accused by Washington of trying to build an atomic bomb, agreed last year to suspend uranium enrichment as a confidence-building measure which helped to persuade the EU to resume trade talks with Tehran last month.

But clouds remain over those talks and parallel discusions focussed on the nuclear issue between Tehran and the EU represented by Britain, France, Germany and Solana.

Last weekend Iran's supreme leader Ayatollah Ali Khamenei warned European powers that they must take their nuclear negotiations with Iran seriously, otherwise Tehran would reconsider its cooperation.

"The Europeans negotiating with Iran should know that they are dealing with a great, cultured nation... if Iranian officials feel that there is no seriousness in the European negotiations, the process will change," Khamenei was quoted as saying by the Iranian media.

The Iranian vice-president, received by Solana in his Brussels office, called for the nuclear discussions to be speeded up.

Solana's spokeswoman said the next meeting on the subject is expected to be held in March in Tehran, bringing together the so-called EU-3's foreign ministers with Solana and Iran's top nuclear negotiator Hassan Rowhani.

"These are very complex issues, we have to look them with great details and we want this to be fruitful," said Gallach.

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