What else is going on in Vienna?

Amid the Viennese chocolate, churches and chamber music, advocates and opponents of the emerging nuclear deal are out in force, all hoping to take advantage of the convergence of world-power officialdom and media to press their points home.

Some of the families of Americans detained or missing in Iran are also making their voices heard.

"We want the Iranian and US governments to know that we are here," said Sarah Hekmati, whose brother, former US Marine Amir Hekmati, is spending his 1,400th day in an Iranian prison on Monday.

"This is an opportune time, and we want to leverage the opportunity."

Also in Vienna is Ali Rezaian. He's the brother of Iranian-American Jason Rezaian, the Washington Post reporter who is facing espionage charges in Iran. 

Along with Hekmati and Rezaian, Christian pastor Saeed Abedini is another American imprisoned in Iran, while retired FBI agent Robert Levinson disappeared in the country eight years ago.

American diplomats have frequently raised the fate of all four US citizens in talks with their Iranian counterparts, but, as yet, there seems little progress on any front.

In the lobby of the Marriott, across from the negotiations at the Palais Coburg hotel, sits Kelsey Davenport, the director for nonproliferation at the Arms Control Association.

Her group "supports a verifiable, comprehensive nuclear deal" based on a framework world powers and Iran hashed out in Switzerland almost three months ago.

Cementing the pact "would be a net-plus for non-proliferation," said Davenport, who arrived in Vienna a week before the June 30 deadline.

This is an opportune time, and we want to leverage the opportunity. - Sarah Hekmati, brother of American imprisoned in Iran

"We are here to follow and report on developments in the talks, consult with parties on both sides of the negotiating table and provide analysis for the media."

The Israeli lobby on alert

Closer to the heart of the closed-door nuclear negotiations, pro-Israel groups opposing an agreement compete with arms control and other organisations for the time and attention of negotiators, journalists and experts in hotel lobbies, bars and restaurants across the city.

The agreement, if reached, would curb Iran's nuclear programme for a decade in exchange for tens of billions of dollars in sanctions relief.

"It's very important to provide context to what is happening," said Trita Parsi, president of the pro-agreement National Iranian American Council, who arrived in Vienna on Saturday.

"The organised elements that don't want this to happen here are very well-funded and have 35 years of experience. A deal is not going to happen or succeed if only one side is represented. There needs to be a counterbalance," Parsi added.

His reference to an opposition is clear. On the other side are opinion-shapers such as Josh Block, the CEO of The Israel Project, and his senior aide Omri Ceren, well-known in Washington for almost daily ruminations on the nuclear talks that are sent to some 2,000 journalists, policy experts and congressional aides.

"We're here to provide facts and analysis, and one fact is that the more details Americans learn about the developing pact, the more people oppose it," Block claimed.

He called the package "the wrong deal for America, our values and the future of the free world".