Iranian Foreign Minister Mohammad Javad Zarif said on Monday he saw a good chance of reaching a final agreement with six world powers on Iran's nuclear programme by a June 30 deadline or a few days later, provided there was political will.

Iran and the six powers - Britain, France, Germany, China, Russia and the United States - are trying to clinch a deal to restrict Iran's nuclear programme for at least a decade in exchange for relief from sanctions.

Such a deal would end a 12-year nuclear standoff between Iran and the West, which suspects Iran is trying to develop a nuclear weapons capability. Iran denies that, saying its programme is for peaceful uses only.

"I believe ... that if there is political will to accept the realities and move forward based on what we agreed in Lausanne, then there is a good possibility we can finish this by the deadline, or a few days after the deadline," Zarif said after talks with EU foreign policy chief Federica Mogherini and the foreign ministers of France, Germany and Britain.

He was referring to a framework agreement reached in Lausanne, Switzerland, in April.

Zarif held separate talks in Luxembourg with the foreign ministers of Britain, France and Germany and then they all met together with Mogherini.

"We discussed how we can expedite the discussions in Vienna for the next few difficult days," he said.

Iranian news agencies quoted Zarif earlier as saying it was worth missing the June 30 deadline by a few days to get a good deal.

British Foreign Secretary Philip Hammond said talks at official level were making progress but there were "a number of different areas where we have not reached full agreement".

"We need to see some flexibility on the Iranian side but ... we cannot compromise on the absolute red lines we have. If we do a deal it has to be verifiable," he said.

"We are all agreed that we need to work towards the June 30 deadline next week and we are going to pull all the stops out," he said.

"At some point - later on this week, over the weekend, early next week - ministers will need to gather and get involved ... We will carry on making progress at official level as far as we can before we involve ministers," Hammond said.

Earlier, Hammond had said he had always expected the talks to go "right to the line and maybe beyond the line".

After the talks the White House spokesman Josh Earnest said the deadline is now "firm" but may be pushed back if needed.

One of the most sensitive points is the powers' demand that any agreement contain a strong verification mechanism to ensure Iran complies with the accord, including at military sites.

French Foreign Minister Laurent Fabius said, "France wants an agreement, but a strong agreement, not a bad agreement."

Such an accord would include a time limit on Iran's nuclear research and production capabilities and an advanced regime to verify Iran's compliance with the agreement, including "if necessary" at military sites, he said.

It should also include the automatic return of sanctions if Iran violates its commitments, Fabius said.

"All that is important both for regional security and for action against nuclear proliferation," he said.

German Foreign Minister Frank-Walter Steinmeier said the talks were entering a "very decisive period".

"We must hope that the Iranians move on decisive points and that we come to a close," he said. – Reuters