Iranian and US officials in Vienna for a fourth round of nuclear talks cautioned Tuesday that there was still hard work to be done before a final deal can be reached.

Arriving in Vienna for the negotiations aimed at drafting the text of an accord ahead of a July 20 deadline, Iran's foreign minister said "a lot of effort" was still needed.

"If there are differences of opinion, which definitely exist, we will spend time to resolve them," Mohammad Javad Zarif told Iranian media after touching down in the Austrian capital.

A senior US official said that the talks between Iran and the five permanent members of the UN Security Council plus Germany would be "very, very difficult."

"There are a range of complicated issues to address and we do not know if Iran will be able to make the tough decisions they must to assure the world that they will not obtain a nuclear weapon and that their programme is for entirely peaceful purposes," the official said.

She added that optimism raised in some quarters that a deal was within reach has "gotten way out of control".

The six powers and Iran want to transform an interim deal struck in Geneva in November into a permanent one by the time a six-month freeze of certain nuclear work by Iran ends on July 20.

In a nutshell the powers want Iran to reduce the scope of its nuclear programme to make it practically impossible for Iran to make a nuclear weapon undetected.

In return the Islamic republic, which denies wanting the bomb and says its aims are purely peaceful, wants all UN and Western sanctions lifted.

If the negotiators can manage to get a deal, this could finally resolve a standoff that has been simmering and threatening to escalate into conflict for the past decade.

- Tall order -

Turning the Geneva deal into something permanent is a tall order, however, particularly with hardliners in the United States, Iran and Israel watching closely.

One major issue, the Arak reactor, appears to have been resolved, with Iran indicating the design could be modified to ease concerns that it could produce weapons-grade plutonium.

But others, most notably uranium enrichment and the sequence of sanctions relief "could be harder to bridge," Kelsey Davenport from the Arms Control Association told AFP.

Enriching uranium -- increasing the proportion of a fissile isotope using supersonic spinning machines called centrifuges -- makes it suitable for peaceful uses, but at high purities it can be used in a nuclear bomb.

Iran already has enough of low-enriched material for several bombs if it decided to "break out" and use its 20,000 centrifuges to enrich this stockpile to weapons-grade.

"Discussions on enrichment are and will be difficult," French Foreign Minister Laurent Fabius told an American Jewish lobby group, AJC Global Jewish Advocacy, on Monday.

- 'Disappointing' -

Other hurdles include Iran's development of ballistic missiles and long-standing questions from the IAEA about "possible military dimensions" to Iran's nuclear work in the past.

A meeting on Monday between the IAEA and Iran on this made little headway, with the atomic watchdog saying that while Iran has taken "several actions" in other areas "work continues".

Some progress was made last year when Iran promised to clarify its need for Exploding Bridge Wire (EBW) detonators, which could theoretically be used in a bomb but which also have other applications.

According to diplomats in Vienna, Iran has yet to convince the IAEA on the detonators issue -- which is only the first step -- ahead of a Thursday deadline.

"If things were on track, the agency and Iran would have agreed, or would be on the cusp of agreeing, the next round of measures," one Vienna diplomat told AFP.

"It would seem at the moment that that hasn't been arrived at yet, which is disappointing if that is the case."