Iran is trying to avoid detailed commitments. The French are sticking to their tough line. And US President Barack Obama faces a battle to sell any deal to a skeptical Congress.

Despite those and other obstacles, negotiators appear increasingly likely to clinch an historic deal to restrict Iran's nuclear program for at least a decade in exchange for relief from sanctions, Western and Iranian officials said.

US officials, including Obama, have long said they see at best a 50-50 chance of getting a deal with Iran.

That remains the official line, but diplomats close to the talks tell Reuters the chances are higher than that as foreign ministers and other negotiators head to Vienna next week for the final stage of a nearly two-year process.

Driving the cautious optimism, they say, is not so much progress made in overcoming sticking points as the intense political pressure on the US and Iranian delegations to reach a deal that would end the 12-year nuclear stand-off between Iran and the West.

"We can't rule out failure, but ... it seems more likely that we will get something. Not by June 30, but perhaps in the days that follow," a senior Western official told Reuters, referring to the planned deadline for a deal.

Another Western official said: "I think there will be an agreement because the two most important players need it."

Six world powers - the United States, Britain, France, Germany, Russia and China - aim to stop Iran from gaining the capacity to develop a nuclear bomb. In return, they would lift international sanctions that have crippled Iran's economy.

A preliminary agreement reached in early April left major differences for negotiators to bridge, including the verification regime to ensure Iranian compliance with a deal and the timetable for lifting sanctions.

While acknowledging such issues were far from being resolved and could lead to problems and delays, diplomats said there was a willingness to compromise.

Securing a nuclear deal is politically vital for Obama and his Iranian counterpart Hassan Rouhani.

For Obama, detente with Iran and another long-term US foe, Cuba, may be the only major foreign policy achievements within reach, with the Israel-Palestinian peace process frozen and wars raging in Syria and Iraq.

"There will be a deal, Americans need it more than we do. This deal will help both countries," said an Iranian official, who put the chances of a final deal at 70 per cent.

One former senior negotiator for one of the six powers told Reuters the chances were as high as 60-40.

For Rouhani, failure to secure an agreement could lead to his political demise. Iran's foreign minister warned US Secretary of State John Kerry about that earlier this year, saying it would give hardline conservatives who oppose the deal a chance to reassert their authority, Iranian officials said.

"It is very crucial for Rouhani and his camp to clinch this deal," a senior Iranian diplomat told Reuters. "Failure means failure of Rouhani and reforms. It will strengthen the position of his hardline rivals."

Neither Obama nor Rouhani can afford to give too much ground, and yet they must compromise to secure a deal.

Obama must win approval for the deal from the Republican-controlled Congress, where many fear the deal will boost Iran's as a regional power and increase its threat to US allies such as Israel and Saudi Arabia. Officials close to the talks say Obama expects Congress to oppose the deal and is prepared to use his veto to ensure it is not derailed.

On Iran's side, the hard-line supreme ruler, Ayatollah Ali Khamenei, has the final word on a deal. Iranian officials say he supports an agreement.

SIGNS OF COMPROMISE

Signs of compromise are emerging.

Western officials say the Iranian negotiators have indicated that the UN International Atomic Energy Agency (IAEA) will be granted access to military sites and Iranian nuclear scientists as part of the verification regime, even though Tehran has publicly rejected that idea.

Kerry appeared this week to soften Washington's insistence that Iran submit fully to a stalled UN investigation into past research suspected to have been related to nuclear weapons.

"We're not fixated on Iran specifically accounting for what they did at one point in time or another," Kerry said on Tuesday.

US officials had previously said Iran must answer the IAEA's queries and some sanctions relief would depend on it. Tehran says the agency's evidence about past arms-related activities is fabricated and insists its nuclear program is peaceful.

A State Department spokesman said on Wednesday that Kerry's remarks had been misinterpreted and US policy was unchanged.

Reuters reported in November that the six would likely stop short of demanding full disclosure of any secret weapon work by Tehran to avoid killing an historic deal.

The major powers recently settled another major sticking point - the so-called "snapback" of UN sanctions in the event of Iranian non-compliance with the deal. Diplomats have declined to discuss details, and Iran has yet to agree.

One wild card in the negotiations, diplomats say, remains France.

It is the most skeptical of the six powers, and the closest to Israel and Saudi Arabia. They blocked agreement on an interim nuclear accord with Tehran in November 2013 before consenting to it weeks later.

French diplomats say they are determined to keep the pressure on for a good deal.

"Our differences with the Americans aren't just semantic. We're insistent on verification and the follow-up to verification," a senior French official said. "We're not there to make Obama's job more difficult."

Another major difficulty in the talks is likely to be Iran's resistance to specifying details on the technical limits on its future nuclear work, Western diplomats said.

"The Americans and Europeans want to fill in the many blanks but the Iranians don't like that," a Western official said.

"Iran wants to keep things vague. That's a problem." - Reuters