VIENNA — Top diplomats from Iran and six world powers geared up on Tuesday for a crucial round of talks aimed at forging a lasting and potentially historic agreement over Tehran’s nuclear program.

A senior US official said negotiators from the five permanent members of the UN Security Council plus Germany were already in Vienna, preparing for the official start on Wednesday. On Monday, Iranian officials had held talks in Vienna with the International Atomic Energy Agency (IAEA).

However, Iran’s top leader said his nation would “not bow” to pressure from world powers in nuclear talks.

Ayatollah Ali Khamenei was quoted by Iran’s state television as saying the world powers “should know” this. He says the Iranian nation will follow the “correct” path.

In the evening, the powers’ principal negotiator, EU foreign policy chief Catherine Ashton, is due to have a working dinner with Iranian Foreign Minister Mohammad Javad Zarif.

This fourth round of talks in the Austrian capital is aimed at transforming an interim deal struck in Geneva in November into a lasting accord before a July 20 deadline.

The six powers want Iran to reduce the scope of its nuclear program in order to make any attempt to make a nuclear weapon practically impossible and easily detectable.

In return the Islamic republic, which denies wanting the bomb in the first place 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.

“If the odds of the talks collapsing are high, the stakes of failure are higher,” Ali Vaez, Iran analyst at the International Crisis Group, told AFP. “Time is of the essence.”

“We hope the leadership in Tehran has given the entire delegation … instructions making it possible to move forward,” Moscow’s negotiator Sergei Ryabkov told the Voice of Russia.

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

White House spokesman Jay Carney said Monday that the last three rounds were to “understand each other’s positions” and that drafting a deal “presents obviously challenges.”

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 its 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.

Other hurdles include Iran’s development of ballistic missiles and its answers to long-standing questions from the IAEA about “possible military dimensions” to its nuclear work in the past.

A meeting on Monday between the IAEA and Iran appeared to make little headway, with the 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 to resolving it — 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.”