Six world powers and Iran made a "good start" in talks in Vienna towards reaching a final settlement in the decade-old dispute on Tehran's nuclear programme, but acknowledged their plan to get a deal in the coming months was very ambitious.

By late July, western governments hope to hammer out an accord that would lay to rest their suspicions that Iran is seeking the capability to make a nuclear bomb, an aim it denies, while Tehran wants a lifting of economic sanctions.

Wide differences remain on how this could be achieved, although the two sides said on Thursday they agreed during meetings this week in the Austrian capital on an agenda and timetable for the talks on such an accord.

"We have had three very productive days during which we have identified all of the issues we need to address in reaching a comprehensive and final agreement," EU foreign policy chief Catherine Ashton told reporters.

"There is a lot to do. It won't be easy but we have made a good start," said Ashton who speaks on behalf of the six powers - the US, Russia, China, France, Britain and Germany.

Senior diplomats from the six nations, as well as Ashton and Iran's Foreign Minister Mohammad Javad Zarif will meet again on March 17, also in Vienna, and hold a series of further discussions ahead of the July deadline.

Tehran says its nuclear programme has no military aims and has signalled repeatedly it would resist dismantling its nuclear installations as part of any deal.

"I can assure you that no-one had, and will have, the opportunity to impose anything on Iran during the talks," Zarif told reporters after the Vienna meeting.

A senior US official cautioned their discussions will be "difficult" but the sides were committed to reach a deal soon.

"This will be a complicated, difficult and lengthy process. We will take the time required to do it right," the official said, speaking on condition of anonymity. "We will continue to work in a deliberate and concentrated manner to see if we can get that job done."

As part of the diplomatic process, Ashton will go to Tehran on March 9-10.

Although modest in scope, the agreement on an agenda for talks is an early step forward in the elusive search for a settlement in the nuclear dispute. A deal could ward off the danger of a new war in the Middle East, reshape the power balance in the region and open up major new trade opportunities.

For Iran, a halt to years of sanctions imposed by the United States, European governments and the United Nations, would end international isolation and lift its battered economy.

VAST DIFFERENCES

The six powers' overarching goal is to extend the time Iran would need to make enough fissile material and assemble equipment for a nuclear bomb, and to make such a move easier to detect before it became a fait accompli.

They will want to cap uranium enrichment at a low fissile concentration, limit R&D of new nuclear equipment, decommission a substantial portion of Iran's centrifuges used to refine uranium and allow more intrusive UN inspections.

The Austria talks follow a ground-breaking accord between Iran and the six powers in November, under which Tehran suspended its most sensitive atom work until late July in return for modest sanctions relief.

That deal was made possible by the election of relative moderate President Hassan Rouhani last year on a platform of ending the Opec producer's international isolation.

Iran's unfinished heavy water Arak reactor, which could produce plutonium for bombs, and its underground Fordow uranium enrichment plant will be among key sticking points.

"We have begun to see some areas of agreement as well as areas in which we will have to work though very difficult issues," the senior US official said.

The official declined to respond specifically to Iranian suggestions that the country's ballistic missile programme was not up for discussion during any final settlement talks.

"All of the issues of concern to the international community regarding Iran's nuclear programme are on the table," the official said. "And all of our concerns must be met in order to get a comprehensive agreement ... Nothing is agreed until everything is agreed."

Iranian ballistic missile work is banned under UN Security Council sanctions.

Zarif said, according to the official Irna news agency: "Nothing except Iran's nuclear activities will be discussed in the talks with the (six powers) and we have agreed on it".

A US delegation will be visiting Israel and Saudi Arabia shortly to discuss the negotiations with Iran, the US official. Both countries have expressed concerns about signs of a possible rapprochement between Iran and the West. -Reuters