VIENNA  - Iran nuclear talks entered the decisive endgame Thursday with a final round of negotiations which could go right down to a July 20 deadline for a deal.

The accord being sought by Iran and the five permanent members of the UN Security Council, plus Germany, would finally ease fears of Tehran obtaining nuclear weapons and silence talk of war.

In exchange, punishing sanctions on the Islamic republic would be lifted.

With insurgents overrunning large parts of Iraq, and Syria in chaos from three years of civil war, a deal could help Tehran and the West normalise relations at a particularly explosive time in the Middle East.

“In this troubled world, the chance does not often arise to reach an agreement peacefully that will meet the needs of all sides, make the world safer, ease regional tensions and enable greater prosperity,” US Secretary of State John Kerry said this week. The so-called P5+1 powers have proposed to Iran a “series of reasonable, verifiable and easily achievable measures”, he said, warning Iran not to “squander a historic opportunity”. “What will Iran choose? Despite many months of discussion, we don’t know yet.”

Iranian Foreign Minister Mohammad Javad Zarif, in a video message, called the talks a “unique opportunity to make history”, saying success would allow both sides to address “common challenges” such as Iraq. But with major differences apparent after five rounds of talks seeking to secure a deal by July 20 - when an interim deal from November expires - Zarif said in French daily Le Monde that some among the P5+1 were suffering from “illusions”.

The six powers want Iran to drastically reduce its nuclear activities in order to render any Iranian drive to assemble an atomic bomb all but impossible.

This would include Iran slashing its capacities to enrich uranium, a process that produces nuclear fuel but also, at high purities, the core of a nuclear weapon.

In itself, that would represent a concession to Iran, which is already defying UN Security Council resolutions ordering it to suspend all enrichment.

But Iran insists it has made too many advances in uranium enrichment to turn the clock back and that it needs to expand its programme in order to fuel a future wave of power reactors.

Demands that Iran’s programme be “radically curbed” rest on a “gross misrepresentation of the steps, time and dangers of a dash for the bomb”, Zarif said.

Francois Nicoullaud, former French ambassador to Iran, said that both sides will need to give ground on this vital issue.

In theory, the July 20 deadline could be extended by up to six months, and many analysts believe this is already being negotiated.

But US President Barack Obama, facing midterm elections in November, is wary of doing anything that could be construed by Republicans as giving Iran more time to get closer to having the bomb.

This is the long-standing accusation of Israel, the Middle East’s sole if undeclared nuclear-armed state which - together with Washington - has not ruled out military action.

Michael Mann, spokesman for EU foreign policy chief and the six powers’ lead negotiator, Catherine Ashton, told reporters he was “not aware” that an extension was being discussed.

“The atmosphere is as always very workmanlike ... (Negotiators) come here with determination to push the process forward and reach a deal by July 20,” he said.