They are among the world's most popular websites and are vital diplomatic channels for Iranian officials involved in the nuclear talks. Yet Twitter, Facebook and YouTube remain banned in Iran.

While Foreign Minister Mohammad Javad Zarif leads the way on social media -- his Twitter account has "Verified" blue tick status -- he is far from alone in seeing its benefits.

Aides to President Hassan Rouhani tweet regularly on his behalf, as do officials from supreme leader Ayatollah Ali Khamenei's office -- most recently to restate Iran's "red lines" for a nuclear deal.

This happens despite Twitter, Facebook and YouTube being among hundreds of sites that ordinary Iranians cannot access unless they install illegal software on their computers, smartphones and tablets.

Iranian authorities filter access, citing inappropriate content as the reason for censorship. The restrictions are widely seen as outdated -- Rouhani wants to remove them and last year vetoed a plan to filter the hugely popular WhatsApp messaging service.

Internet use is growing every year, with 45 million of Iran's 78 million population online regularly. The contradictions of censorship policies "no longer raise eyebrows, except those belonging to foreigners," according to Hooman Majd, an Iranian-American journalist and author of several books on modern Iran.

"Any sophisticated politician -- and Zarif is certainly sophisticated -- understands that social media plays a big part in establishing a narrative," he said, noting that the minister uses Facebook in Farsi to message Iranians and Twitter in English to match diplomatic counterparts such as US Secretary of State John Kerry.

"I don't think he thinks too hard about the contradiction," Majd, who is covering the epic talks in Vienna for America's NBC News network, said of Zarif.

- Bid to 'win the public mind' -

"It is widely recognised, and conceded by the authorities, that Facebook has millions of Iranian members," he added.

As if to prove Twitter's usefulness again, Zarif used it Thursday just moments before Kerry told reporters he was prepared to "call an end" to the talks in Vienna if "tough decisions" are not made.

"We're working hard, but not rushed, to get the job done. Mark my words; you can't change horses in the middle of a stream," Zarif tweeted, adding the main #IranTalksVienna hashtag.

Online video sharing has also played a part. At talks on July 3, Zarif used YouTube to set out Iran's position in the talks, just as he did before an earlier round of negotiations in 2014.

Last week's video extended a promise of greater cooperation from Iran on global problems such as the Islamic State group, if a nuclear deal were reached.

In English, Zarif said: "Getting to yes requires the courage to compromise, the self-confidence to be flexible, the maturity to be reasonable. Our common threat today is the growing menace of violent extremism and outright barbarism."

Such direct diplomacy complements use of traditional media. To get his message across, Zarif has written columns for The New York Times and Financial Times urging a deal.

By doing so he can shape the debate, said Abas Aslani, a well-known Iranian journalist covering the nuclear talks in Vienna.

"Often in politics the narrative of an event is much more important that the event itself," said Aslani, acknowledging that cheap and easily available VPNs (Virtual Private Networks) neuter censorship.

"Those who play a more active and influential role in different kinds of media are expected to win the public mind," he said of Zarif.

"The presence of Iranian officials in media, traditional and new media, not only shouldn't be limited but also expanded."