The last round of talks towards a comprehensive nuclear agreement between the world powers and Iran began this week amid statements from both sides that put their official deadline in doubt.

While a deal is supposed to be completed by June 30 , it is now widely expected that the talks will extend beyond that.

The talks in Vienna were preceded on Monday by a meeting between Iran's foreign minister Mohammed Javad Zarif and his counterparts from Britain, France and Germany, along with European Union foreign policy chief Federica Mogherini. They were followed by a round of talks at a lower level of deputy ministers on Tuesday. The foreign ministers will rejoin the talks this weekend.

While the broad outline of the deal was presented in Lausanne three months ago , a speech by Iran's Supreme Leader Ali Khamenei in Tehran underlined the main obstacles still remaining.

“All economic, financial and banking sanctions, implemented either by the United Nations Security Council, the United States Congress or the administration, must be lifted immediately when the deal is signed,” said Ayatollah Khamenei, contradicting the Lausanne statements - at least as they were presented by the western side. The P5 + 1 group had made clear that the sanctions would be removed gradually, in accordance with Iran's compliance with International Atomic Energy Agency (IAEA) inspections.

However, the Supreme Leader objected to these inspections as well, saying in his speech that Iran would not allow “unconventional inspections, interrogating certain Iranian individuals and inspecting military sites.”

Khamenei also denied another central component of the deal, a long-term moratorium on Iran's nuclear research. He said: “Freezing Iran’s research and development for a long time, like 10 years or 12 years, is not acceptable.”

The Supreme Leader's speech - in which he also expressed his confidence in the Iranian negotiating team headed by Mr Zarif - may have been an attempt to mollify hardliners in the Iranian regime opposed to the talks.

Israeli officials certainly believe that the West's willingness to reach an agreement, particularly that of the Obama administration and its Secretary of State John Kerry, will push the negotiators to find "creative" ways to overcome the obstacles. "That there is going to be deal at this stage is almost a given," said one senior Israeli security official. "The question is only how stringent the restrictions it will place on Iran."

Once the deal is signed, the US Senate will have 60-day period in which to review it and vote on it. Opponents will make use of the State Department's own annual report on terrorism, published last week, which said: “In 2014, Iran’s state sponsorship of terrorism worldwide remained undiminished."

However, as things stand in the Senate now, most Democratic members are not planning to challenge President Barack Obama on his signature foreign policy initiative and a two-thirds veto-proof majority against the agreement is unlikely.