Senior officials from France, Britain and Germany will meet Iran's top nuclear negotiator in Vienna today to offer Tehran a final chance to halt uranium-enrichment plans or face possible UN sanctions.
"This Thursday, there will be a meeting of our political directors," British Foreign Secretary Mr Jack Straw told a London news conference with German Foreign Minister Mr Joschka Fischer.
"What will be sought on Thursday will be discussions about Iran's compliance - not with any conditions laid down by the three of us, but by the (International Atomic Energy Agency) board of governors," he said. "A proposal will be put to them."
Last month, the IAEA board passed a resolution demanding that Iran freeze its uranium-enrichment activities - procedures that could produce fuel for nuclear weapons - but Tehran rejected the demand as illegal.
Officials in Tehran said negotiator Mr Hassan Rohani, secretary-general of Iran's Supreme National Security Council, would travel to Italy this evening.
It was unclear what his itinerary would be prior to Thursday's meeting in Vienna. Mr Gholamreza Aghazadeh, head of Iran's Atomic Energy Organisation, said Iran was determined to press ahead with its atomic plans and would not give up its right to enrich uranium.
Tehran says it wants to produce low-enriched uranium fuel only for power generation, but Washington says it has a covert plan to produce highly enriched fuel for atomic weapons.
The IAEA has been investigating Iran's nuclear programme for more than two years. Although it has uncovered many previously hidden activities that could be related to a weapons programme, it has found no "smoking gun" to support US allegations Tehran has an active atom bomb programme.