A July 20 deadline for Iran and world powers to strike a nuclear deal looks increasingly likely to be extended given the seemingly huge gap that remains between the two sides, experts say.

Though the aim is still to reach agreement by the stated deadline, officials familiar with the talks admit a delay is looking more and more possible.

"I imagine those conversations have already started," Ray Takeyh, a former senior advisor on Iran at the US State Department, told AFP.

"It seems likely given all the news about impasses and stalemates," added Takeyh, now a senior fellow at the Council on Foreign Relations.

Mark Fitzpatrick, also formerly of the US State Department and now with the International Institute for Strategic Studies, agreed.

"I doubt that an extension is being formally discussed, because that would be to admit failure to meet the July 20 deadline," he said. "But some discussion of it must be underway informally."

Iran and the five permanent members of the UN Security Council plus Germany want to strike a lasting nuclear deal by the time a six-month interim agreement agreed in November expires on July 20.

Under this mooted deal, the powers want Iran to reduce substantially in scope its nuclear programme in order to make it all but impossible for the Islamic republic to make an atomic bomb.

In return Iran, which denies it is seeking to develop nuclear weapons, wants painful Western and UN Security Council sanctions lifted.

Such an historic but fiendishly complex deal could silence once and for all threats of conflict in the Middle East, and set Iran and the West on the path to normalised relations.

After three rounds of talks in Vienna that appeared to go well, the process clearly hit the skids last month, with both sides talking of major differences.

The main sticking point, experts say, is Iran's refusal to bow to Western demands to slash its capacities to enrich uranium, a process which can produce nuclear fuel but also the fissile core of a atomic weapon.

Tehran argues that it needs these capacities to provide fuel for a future fleet of nuclear plants. Western powers counter that Tehran is years, even decades, away from having these plants, and therefore has no need to enrich uranium on such a scale.

In an effort to inject some momentum before a new round of talks next week, Iran held a series of bilateral discussions, including with US negotiators in Geneva.

It was unclear whether any breakthrough was made during these talks, although Russia's pointman Sergei Ryabkov said after meeting Iranian negotiators in Rome he was "relatively optimistic".

"As far as we can tell, the Iranian delegation has done important work with our American and French colleagues," Ryabkov told Russian news agencies.

- Mutual consent -

The possibility of an extension has always been an option. November's interim deal states that the six-month period was "renewable by mutual consent".

Mark Hibbs, of the Carnegie Endowment for International Peace, believes that some Western officials are ready to extend -- but at a price.

"They are only willing if between now and July 20 the Iranians show that they are serious, that they will make substantial concessions and come down from hardline positions," Hibbs told AFP.

"The powers and Iran have been drafting documents in preparation for an eventual extension for a long time."

But at the same time, Hibbs said that US President Barack Obama would prefer to get a deal now -- before midterm US elections in November expected to produce a Congress "much more critical" of Iran.

Simply rolling over the six-month November interim deal is no simple matter, meanwhile, Hibbs said.

That deal included a suspension of some Iranian nuclear work and some sanctions relief. But it also involved one-off measures, such as unfreezing billions of dollars and Iran neutralising nuclear material.

In addition, extending the deadline may risk the ire of Israel and of hardliners in Washington who already see the talks as a way for Iran to buy more time and get closer to having the bomb.

Iranian President Hassan Rouhani is also under pressure at home to get a deal as soon as possible, although according to Siavush Randjbar-Daemi, an Iranian lecturer at Manchester University, he could sell an extension at home.

"Rouhani still commands the support of the supreme leader for the moment," he said.

Kelsey Davenport, analyst at the Arms Control Association, believes that at present it is "premature" to discuss an extension.

But she warns that neither side should flinch from doing so -- if it becomes necessary.

"Extending the talks in order to reach an agreement is a far better scenario than failure, which would result in an unconstrained Iranian nuclear programme and possible military action," she told AFP.