VIENNA - US Secretary of State John Kerry said Thursday that world powers were not discussing extending a looming deadline for a nuclear deal with Iran despite being “concerned” about the remaining gaps.

Iran meanwhile showed no sign of weakening its position, four days before the cut-off point for Iran and the five permanent members of the UN Security Council plus Germany to get an accord.

“We are not discussing an extension. We are negotiating to have an agreement. It’s that simple,” Kerry said in Paris before leaving for the Austrian capital to join the discussions.

He added though that together with British Foreign Secretary Philip Hammond, who said Wednesday he was “not optimistic” and suggested an extension was the best they could hope for, he was “concerned about the gaps”. “We all are,” Kerry said. Iran and the six powers have been negotiating intensively since February to turn an interim accord with Iran reached a year ago into a lasting agreement before November 24.

Such a deal, after 12 years of rising tensions, is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities - an ambition the Islamic republic has always denied. Iran’s nuclear chief on Thursday ruled out further negotiations on the design of a reactor that the West fears could be used to produce plutonium for an atomic bomb.

The future of the Arak reactor is one of the main issues in talks between Iran and six world powers under way in Vienna aimed at striking a deal on Tehran’s nuclear programme by November 24.

Iran insists the unfinished reactor, located 240 kilometres (145 miles) southwest of Tehran, is solely for research purposes. It has already promised to make some modifications to the design of the heavy water reactor to limit plutonium output. The United States has proposed transforming Arak into a light water reactor so that it produces far less plutonium, but Tehran has refused. “On Arak, we have said we were ready to design it so that the concerns are lifted. This matter is settled to some extent on the technical aspect and there is no more room for further negotiations,” Ali Akbar Salehi was quoted by local media as saying. The parties missed an earlier deadline of July 20, giving themselves another four months, until November 24, to thrash out an agreement.

Some areas appear provisionally settled in what would be a highly complex deal that would run for many years, even decades. But two key issues remain: enrichment - rendering uranium suitable for peaceful uses but also, at high purities, for a weapon - and the pace of the lifting of sanctions. Diplomats say Iran wants all sanctions lifted at once. The six world powers want however to stagger any suspension to be sure that Iran won’t renege on its commitments. Iran wants to massively ramp up the number of enrichment centrifuges - in order, it says, to make fuel for a fleet of future reactors - while the West wants them dramatically reduced.

Iran’s nuclear chief Ali Akbar Salehi on Thursday stuck to this position, saying Iran would increase its enrichment capacity to 190,000 SWU (Separative Work Units) - around 20 times its current processing ability - within eight years.

The six powers say Iran has no such need in the foreseeable future. Russia is contracted until 2021 to fuel Iran’s only power reactor at Bushehr and last week signed a deal to build - and fuel - several others.

Salehi also said the much-reported idea of exporting its stockpile of low-enriched uranium - enough for around eight bombs if purified to weapons grade - “makes no sense”. Upping the ante, Iran’s President Hassan Rouhani said on Wednesday that a deal was possible but only if the six powers did not ask for too much.

“If the other side shows the political will to reach an accord and doesn’t make excessive demands, a deal could be done,” he said on his website.

Kerry was due in Vienna on Thursday evening but it was unclear when other foreign ministers - Iran’s is already there - might arrive.

Russian Foreign Minister Sergei Lavrov, whose country is a crucial player in the talks, will only attend if there is sufficient progress, Moscow’s lead negotiator Sergei Ryabkov told Russian media this week.

“With the arrival of Kerry the talks will move into a more serious direction,” RIA Novosti cited a source as saying. “Much depends on what decisions he will bring from Washington”.

Lavrov was due to meet his Saudi counterpart in Moscow on Friday, and therefore cannot be in Vienna before Saturday, a foreign ministry spokesman said in Moscow on Thursday.

The Sunni Muslim powerhouse and other Gulf monarchies are watching the talks with a sceptical eye, as is Israel, the Middle East’s sole if undeclared nuclear-armed state.

Israel’s intelligence minister Yuval Steinitz pressed the powers Thursday not to sign a “bad deal with Iran that enables Iran to remain a threshold nuclear state”. “A deal is still possible by November 24,” Arms Control Association analyst Kelsey Davenport told AFP. “The remaining obstacles can be overcome if both sides are willing to show some flexibility.”