Iran and world powers begin a new round of nuclear talks Tuesday hoping to make enough progress to move up a gear and start drafting a historic final deal next month.

This will however require Iran and the five UN Security Council permanent members plus Germany to delve ever deeper into issues that will severely test their willingness to give ground.

Both sides want to transform an interim deal struck in November by foreign ministers in Geneva into a permanent agreement by the time this temporary accord lapses on July 20.

So far the mood music has been good, with the powers' chief negotiator, EU foreign policy chief Catherine Ashton, hailing the last monthly round in mid-March as "substantive and useful".

"There are signs that an understanding is possible that respects the rights of the Iranian nation," Iranian Foreign Minister Mohammad Javad Zarif told Iranian media afterwards.

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.

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.

This 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.

One issue is proving to be particular thorny -- that of Iran's desire to research and develop newer and faster centrifuges, something which November's deal allowed them to continue, one diplomat said.

- Hardliners -

Any deal that gives 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 of Iran's nuclear infrastructure intact would also be a hard sell to sceptical US lawmakers and to Israel, the Middle East's sole if undeclared nuclear power.

Threatening to blow the whole process out of the water by driving a wedge between Russia and the United States and other Western powers is the crisis over Moscow's annexation of Crimea.

Russia's chief negotiator, 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," Ryabkov told Interfax.

"But if we are forced, here we will take the path of counter-measures, because when it comes down to it, the historic value of what has happened (over Ukraine) ... is incomparable with what we are doing" on Iran.

Moscow and Iran are said to be negotiating a oil-for-goods deal that would undermine Washington's sanctions efforts, a strategy it credits with getting Iran to talk in the first place.