The final round of talks in Vienna on Iran's nuclear programme, which were originally slated to end on Tuesday, have been extended by seven days, as new "red lines" drawn up by Iran's Supreme Leader have greatly complicated matters.

The difficulty of reaching a deal was underlined this week when it emerged that both International Atomic Energy Agency (IAEA) Director General Yukiya Amano and Iranian Foreign Minister Mohammed Jawad Zarif had both made unscheduled trips to Tehran.

Supreme Leader Ali Khamenei announced two weeks ago that Iran would not allow inspections of its military sites by IAEA inspectors - and Mr Amano's surprise trip to Tehran indicated that the statement has been a major stumbling block.

IAEA verification is a key element of any agreement. Ayatollah Khamenei also demanded that sanctions on Iran be removed immediately after a comprehensive agreement is signed, and that Iran be allowed to continue nuclear research and development - despite having agreed to a moratorium of at least ten years in the Lausanne framework three months ago.

It is understood that Foreign Minister Zarif returned to Tehran earlier this week in order to decide on a clear set of parameters for negotiating the final deal.

Despite the obstacles, the atmosphere at the talks in Vienna was, outwardly at least, upbeat, with diplomats on both sides saying that they were never nearer signing a deal.

The assumption in Israel's diplomatic and intelligence community is also that an agreement is likely.

The chief negotiator representing the international community, US State Secretary John Kerry, said "we had a good conversation" after a meeting with Mr Zarif on Tuesday.

However, more sombre notes came from Washington, where American President Barack Obama said: "I will walk away from the negotiations if it's a bad deal… ultimately this is going to be up to the Iranians."