According to media reports citing diplomats, the United States and European Union are likely to demand its immediate closure. 

“It was constructed essentially in secret by the Iranians and unearthed by the West in September 2009. What’s happened more recently is that the facility has essentially come online and has started actually producing enriched uranium,” said David Cliff, a nuclear proliferation expert at the non-governmental organization, Vertic.

The meeting in Istanbul includes the five permanent members of the U.N. Security Council - the U.S., Britain, China, Russia and France - plus Germany, and Iran.

Tehran denies Western accusations that it is trying to make a nuclear bomb. It says the enrichment program is for producing electricity. It has promised new initiatives at the talks - though it has not specified what it might offer.

The U.S. and the EU have imposed a tightening set of sanctions on Iran - most recently, banning oil imports.

Analysts say Tehran can depend on China and Russia for limited support. Beijing has called for the P5+1 to take a "maneuverable and pragmatic attitude."

The Iranian leadership itself is divided, says Cliff. “There’s not been a decision taken within Iran as to whether they want to go for a bomb outright or whether they want to develop what is called a ‘virtual nuclear weapons capability,’ where they have all of the material that they need, and they have all of the scientific and the technical knowledge they would need, to develop a nuclear weapon if at some point in the future the ‘go’ order was given,” he said. 

If that happened, both the U.S. and Israel have said they would have little choice but to launch a military attack on Iran. Analyst Andrea Berger, at the Royal United Services Institute in London, says there is optimism that diplomacy will prevail. 

“With the current negotiations, none of the countries around the table have articulated that this is the end for engagement with Iran. As a matter of fact, the schedule for negotiations seems to imply that countries are hoping there will be future negotiations to come,” Berger said.

In the run-up to the Istanbul talks, tension between the West and Iran has grown as international sanctions tighten against Tehran. With Western patience running thin, Washington warns that time for diplomacy is not “infinite” and that “all options remain on the table.”