Iran and world powers said Wednesday that their nuclear talks were now moving up a gear, with Tehran's foreign minister saying both sides agreed on "50-60 percent" of issues.

Speaking after the latest round of talks in Vienna, the powers' chief negotiator, EU foreign policy chief Catherine Ashton said the next round from May 13 would see negotiations "move to the next phase".

A senior US official said that the next meeting, also in Vienna, would see Iran and the five permanent members of the UN Security Council plus Germany "begin actually drafting the text".

But in a statement repeated by Iran's Foreign Minister Mohammad Javad Zarif, Ashton said a "lot of intensive work will be required to overcome the differences which naturally still exist".

"I can say we agree on 50-60 percent of issues, but the remainders are important ones and diverse," Zarif told Iranian media. "Even two percent can torpedo all of it".

And in a sign of the difficulties ahead, the US official said that one issue remained Iran's ballistic missile programme, which Tehran has said is not up for discussion within the nuclear talks.

Zarif said also that Iran's negotiators have "put forward our stance that none of our (nuclear) facilities would be dismantled", one of the West's probable main demands.

"I would caution everyone from thinking a final agreement is imminent or that it will be easy. As we draft, I have no doubt this will be quite difficult at times," the US official said.

China's envoy Wang Qun said the latest round had been "fruitful" and that the talks were "building momentum".

In November the two sides reached an interim deal under which Iran froze certain parts of its nuclear activities in return for minor relief from painful Western sanctions.

But Iran has not permanently dismantled any of its nuclear equipment and can fully reactivate its facilities if it wishes when the deal expires on July 20, the deadline for agreeing a final deal.

US Secretary of State John Kerry told US lawmakers Monday that the theoretical period needed for Iran to produce a weapon's worth of bomb material -- if it chose to do so -- was "about two months".

In order to greatly extend this "break-out" time, the six powers want the final deal to see Iran reduce permanently, or at least long-term, the scope of its programme.

This may involve Iran slashing the number of centrifuges -- used to enrich nuclear material -- changing the design of a new reactor at Arak and giving UN inspectors more oversight.

Other outstanding thorny issues include Iran's research and development of new nuclear machines, which November's deal allowed the Islamic republic to continue.

Iran's supreme leader Ayatollah Ali Khamenei said Wednesday "that Iran's activities in nuclear research and development as well as its nuclear achievements will never be stopped".

Any agreement will need to be sold to sceptical hardliners both in the United States and Iran as well as to Iran's arch enemy Israel, widely assumed to have a nuclear arsenal itself.

- US-Iran spat -

Threatening to throw a spanner in the works is the crisis over Ukraine which has led to the biggest standoff between Russia and the West since the Cold War.

Russia's chief negotiator, Sergei Ryabkov, having fired a warning shot last month, backtracked Tuesday, telling ITAR-TASS it would "not be wise" to turn Iran into a "bargaining chip".

Wang said that Ryabkov was "utterly constructive".

Moscow and Iran are said to be eyeing an oil-for-goods deal that would undermine US sanctions efforts, which Washington credits with getting Tehran to talk in the first place.

Another issue casting a cloud over the talks is the spat over Iran's selection of a new UN ambassador allegedly linked to the 1979 US hostage crisis, with Washington saying he was "not viable".