Iran and six world powers appeared close to a deal yesterday to give Tehran sanctions relief in exchange for limits on its nuclear programme, but Iranian officials said talks could run past their latest midnight deadline and success was not guaranteed.

In fact by late yesterday there were no concrete signs of a breakthrough as diplomats in Vienna continued to struggle over sticking points such as UN sanctions and access to Iranian military sites.

Earlier, US Secretary of State John Kerry sat in silence when asked if the deadline might be extended or if he could rule out an extension. His Iranian counterpart, Mohammad Javad Zarif, said that “there shouldn’t be any extension”, according to the semi-official Fars news agency, only to add: “But we can continue the talks as long as necessary.”

At one point Zarif spoke with Kerry and European Union foreign policy chief Federica Mogherini for more than an hour.

We can continue the talks as long as necessary

Diplomats said there were contingency plans for an announcement ceremony in the event of a deal, which would open the door to ending sanctions that have crippled Iran’s economy, in exchange for at least a decade of curbs on its nuclear programme.

The Western powers in particular suspect Iran may have sought to use its civil nuclear programme as a cover to develop a nuclear weapons capability. Iran says the programme is solely for peaceful purposes. If no agreement is clinched this time round, the powers will need to extend the terms of an interim nuclear deal that has already been extended three times in two weeks.

Another option is to walk away, something both the Americans and Iranians have said they are willing to do. They could also suspend the talks for a few weeks or months, though Iran has said it opposes this.

Photographers and cameramen try to take pictures and videos after a meeting in Palais Coburg, the venue for nuclear talks, in Vienna, Austria yesterday.

Among the biggest sticking points in the past week has been Iran’s insistence that a UN Security Council arms embargo and ban on its ballistic missile programme dating from 2006 be lifted immediately if an agreement is reached. Russia, which sells weapons to Iran, has publicly supported Tehran on the issue.

Other problematic issues are access for inspectors to military sites in Iran, explanations from Tehran of past activity that might have been aimed at developing a nuclear weapon, and the overall speed of sanctions relief.

Iran’s Fars news agency reported that Iran was pushing for the draft UN Security Council resolution under discussion as part of the deal to state explicitly that Tehran’s nuclear programme is legal.

Comments from both senior Republican and Democratic senators on Sunday suggested that any final deal would also face tough scrutiny in the US Congress.

A senior Iranian official said 99 per cent of the issues had been resolved, adding: “With political will, we can finish the work late tonight and announce it tomorrow (today).”

Israeli Prime Minister Benjamin Netanyahu’s point-man on the Iranian talks, Energy Minister Yuval Steinitz, told Israel’s Army Radio he was expecting an agreement during the night or early today. He reiterated Israel’s opposition to the deal.

“What is being drafted, even if we managed to slightly improve it over the past year, is a bad agreement, full of loopholes,” he said. “If we call it by its true name, they are selling the world’s future for a questionable diplomatic achievement in the present.”