Iran and six world powers ended the opening round of nuclear talks on an upbeat note Thursday, with both sides saying they had agreed on a plan for further negotiations meant to produce a comprehensive deal to set limits on Tehran's nuclear ambitions.

In a joint statement, they said the next round of negotiations would begin in Vienna on March 17, continuing a process likely to take at least six months and probably longer.

Expectations had been modest as the talks started Tuesday, and the positive tone on a framework for future talks appeared aimed in part to encourage skeptics inside and outside Iran that the negotiations had a chance to succeed despite huge gaps between the Iranians and the six powers.

'We have … identified all of the issues we need to address for a comprehensive and final agreement.'- Catherine Ashton, European Union diplomat

Iranian Foreign Minister Mohammad Javad Zarif, who came to the talks vowing Iran would never strip down its nuclear facilities, was smiling and relaxed as he read out the joint statement. But in a message intended for skeptics at home who fear Iran will give up too much at the talks, he told state TV afterward that his nation would "not close down any site."

The six want Tehran to agree to significant cuts in its nuclear program to reduce concerns it could be turned quickly to weapons use.

Iran opposes cuts, saying its program is not aimed at building weapons. The U.S. and its partners say that Iran must come to an agreement if it wants a full end to sanctions crippling its economy.

"We have … identified all of the issues we need to address for a comprehensive and final agreement," said Catherine Ashton, the EU's top diplomat who convened the talks between Iran and the six powers — the United States, Russia, China, Britain, France and Germany.

"It won't be easy, but we've gotten off to a good start," she said in a statement read later in Farsi by Zarif.

A Western diplomat said Ashton would visit Tehran March 9-10 for preparatory talks. He demanded anonymity because his information was confidential.

Final deal tough

From Baghdad, Russian Foreign Minister Sergey Lavrov said Moscow sensed that "all sides are interested in being serious and being pragmatic."

The talks are designed to build on a first-step deal in effect since last month that commits Iran to initial nuclear curbs in return for some easing of sanctions. The deal can be extended by mutual consent after six months. Both sides say any final deal will be tough to reach.

Under the first-step deal, Iran began to carry out a series of steps over six months. They include diluting or converting its stockpile of higher enriched uranium that can be turned quickly into weapons-grade material and not to make any more for the next six months.

Iran also agreed not to increase its stockpile of lower-enriched uranium and not to set up new centrifuges at its enrichment plants as well as to rigorous oversight of the implementation of its commitments by the UN nuclear agency.

Sanctions to be suspended during the interim agreement include those on Iran's petrochemical exports, its trade in gold and precious metals, its car industry and the supply of parts for Iran's civil aviation industry. There will be no new sanctions as long as the first step deal remains in effect.

'Positive step'

In a report on Thursday, the UN International Atomic Energy Agency confirmed Iran was keeping its side of the bargain, except for converting some of its higher-enriched uranium stockpile. Tehran has said it needed more time to set up the technical capacity.

The IAEA commended Iran's "positive step forward" in granting greater access to nuclear facilities. But alluding to the agency's attempts to probe suspicions that Tehran worked secretly on nuclear weapons, the report said "much remains to be done to resolve all outstanding issues."

The six countries want to leave Iran with little capacity to quickly ramp up its nuclear program into weapons-making mode with enriched uranium or plutonium, which can be used for the fissile core of a missile warhead.

They say Iran should dismantle or store most of its 20,000 uranium-enriching centrifuges, including some not yet working. They also want a reactor now being built to either be scrapped or converted from a heavy-water setup to a light-water facility that makes less plutonium.