Appearing closer than ever to a positive agreement, the US, Britain, France, Germany, Russia and China have been working intensely with Iran trying to break an impasse for more than a week. Thousands of hours of diplomatic work have spread over 20 months.

Mohammad Javad Zarif, Iran’s foreign minister, shuttled back to Tehran on Sunday evening, and then returned to Vienna.

At the start of his meeting with US Secretary of State John Kerry, he said: “I am here to get a final deal and I think we can.” That is even though all sides have their red lines.

There have been both full table and bilateral talks. France has appeared as one of the toughest participants. Foreign Minister Laurent Fabius insisted on being able to restore sanctions immediately if Tehran reneged on its obligations under a deal.

“If there is an agreement, there will be a lifting of sanctions, but if the agreement is not implemented, there has to be what we call a snapback mechanism, which automatically will bring