Iran's second enrichment plant is believed to be inside an underground facility
By Paul Reynolds
World affairs correspondent, BBC News website
A new phase in the diplomatic effort to get Iran to freeze its nuclear activities will be opened this week.
Talks are set to resume a week after the revelation that Iran has built a new enrichment plant in the mountains near Qom.
Western diplomats say that the talks represent the last chance Iran has to convince the countries negotiating with it that its intentions are peaceful and that if there is no progress, then further sanctions will be considered by the end of the year.
One official said that "it will leave us in a dangerous place" if nothing had been resolved by then, meaning that the risk of an attack by Israel would have increased.
But doubts are growing that the US will be able to enforce its first choice of new sanctions - a ban on the import to Iran of finished petroleum products.
Such a ban would hit at the weak link in Iran's energy industry. Despite its huge oil reserves, Iran cannot refine enough finished products itself and imports about 40% of them.
US allies appear reluctant to go this far, including French Foreign Minister Bernard Kouchner, who said: "I think this is a bit dangerous." The measure would hit "mainly poor people", he added.
Russia also seems doubtful, despite suggestions last week from President Dmitry Medvedev that more sanctions might be necessary.
The ice in relations between the US and Russia might have been cracked but there is no sign of a real thaw.
Russian Foreign Minister Sergei Lavrov seemed unmoved by news of the second enrichment plant, saying there was "no clarity regarding the legal issues".
This was a reference to the dispute between Iran and the International Atomic Energy Agency (IAEA) about when Iran is supposed to declare any new nuclear facilities.
According to the IAEA, Iran is bound by what is known as the Subsidiary Agreement it reached when its main plant at Natanz was found in 2002. This agreement commits it to disclosure at the preliminary design stage.
Iran says it never ratified that agreement, so says it is committed only to giving notice of 180 days, which it says it did in this case -with a year to spare.
Against this background the new diplomatic phase begins in Geneva on Thursday with direct talks between Iran and the six countries negotiating with it - the US, Russia, China, Britain, France and Germany.
The latter grouping is known either as the P plus 1 (the five permanent members of the Security Council plus Germany) or the E3 plus 3, the three Europeans countries plus the others.
The talks are the fruit of US President Barack Obama's new "extended hand" approach and the US will be taking part in the talks as a full participant for the first time.
The P5 plus 1 is insisting that Iran freezes its uranium enrichment to allow for talks on a package of aid that includes help with nuclear power generation. Iran is refusing any freeze as a precondition and has offered talks on global issues, including nuclear disarmament.
The prospects are not good. Iran is unlikely to change its mind suddenly and is currently showing a determination to press on with its ballistic missile programme by staging tests of its two of most advanced rockets, the Shahab-3 and the Sajjil, the latter being solid-fuelled and therefore more mobile.
If the US cannot get agreement on a ban on refined petroleum products, it is likely to rely on stopping investment in Iran's oil and gas industries and on further financial restrictions on Iranian banks and financial transactions.
It hopes that such action would be authorised by the Security Council, as has happened three times before.
But if Russia - and China, which has a huge long-term deal to buy Iranian oil and gas - says no, the US will try to rally its allies, especially the EU, to do it anyway.