VIENNA (AFP) - Iran and world powers embarked on a new round of nuclear talks on Tuesday hoping to make enough progress to move up a gear and start drafting a historic final deal next month.
Threatening to drive a wedge between the powers, however, is the crisis over Ukraine, which has led to the biggest standoff between Russia and the West since the Cold War.
Iran and the five UN Security Council permanent members plus Germany want to transform a deal struck in November into a permanent agreement before this temporary accord lapses on July 20.
Doing so is a tall order, however, requiring both sides to tackle thorny issues that will severely test their willingness and ability to give ground to the limit.
Iranian Foreign Minister Mohammad Javad Zarif said on Tuesday that he has formed a group of legal experts to help in the "complicated, difficult and slow work" of drafting a deal.
A spokesman for the powers' chief negotiator, EU foreign policy chief Catherine Ashton, said this round, expected to last two days, was to "explore our respective positions on each topic".
Ms Ashton and Mr Zarif held a working dinner on Monday evening, her spokesman added.
A senior US official involved in the talks said Friday she was "absolutely convinced" a deal could be reached and that both sides were "looking toward beginning drafting (a deal) in May".
But "the real issue is not about whether you can write the words on paper, do the drafting. It's about the choices that Iran has to make, and some of them are very difficult."
Under the November deal, which took effect on January 20, Iran froze certain nuclear activities for six months in exchange for minor relief from sanctions hurting its economy.
As part of the deal, Iran was given greater access to civilian aircraft parts and on Monday Washington said that Boeing had been issued with a temporary licence to do business with the Islamic republic.
Now the powers want Iran to reduce permanently, or at least long-term, the scope of its programme in order to make any dash to make the bomb extremely difficult and easily detectable.
Iran in return wants all sanctions lifted.
The deal may involve Iran slashing the number of centrifuges - machines "enriching" nuclear material - changing the design of a new reactor at Arak and giving UN inspectors more oversight.
Any deal that sees Iran giving too much away risks losing Iranian President Hassan Rouhani - who since taking office last year has sought to improve ties with the West - or the supreme leader.
But leaving too much - or indeed any - of Iran's nuclear infrastructure intact would also be a hard sell to skeptical US lawmakers, and to US ally Israel, the Middle East's sole if undeclared nuclear power.
So far, the six powers have shown a united front. But Moscow's annexation of Crimea last month has sent relations between Russia and the West into a tailspin.
Following the latest unrest in the east, with pro-Russian activists declaring independence, the White House on Monday warned Moscow against efforts to "destabilise Ukraine".
The Kremlin swatted the accusations aside, warning Tuesday the pro-Western government in Kiev against any use of force and saying there was a "risk of unleashing civil war".
Russia's chief negotiator, Mr Sergei Ryabkov, fired a warning shot last month, saying Moscow might alter its position on the Iran talks if pushed too far.
"We would not like to use these talks as an element of a stakes-raising game," Mr Ryabkov said. "But if we are forced, here we will take the path of counter-measures."
Moscow and Iran are said to be negotiating a oil-for-goods deal that would undermine Washington's sanctions efforts, a strategy the US credits with getting Iran to talks in the first place.