Kamal Kharazi said, however, that the start of the talks which opened on Monday in Brussels augured well because of their seriousness and "determination to reach an agreement."

Monday's talks were focused on giving Tehran the trade, technology and security rewards in return for suspending the nuclear activities under an an agreement struck last month in Paris.

"We hope they go quickly and work seriously in this phase of discussion," Kharazi said at a joint press conference with visiting South African counterpart Nkosazana Dlamini-Zuma in Tehran.

He mentioned several times a forecast of three months for an agreement, a timeframe questioned by the European side.

"If the talks have been positive and we have obtained that to which we have the right, we will continue. IF we have wasted our time, we will take our decision," he added.

Iran's top nuclear official Hassan Rowhani warned on Sunday that Iran reserved the right to break off talks at any time if they did not progress.

Rowhani, speaking on Monday after his first talks with the foreign ministers of Germany, France and Britain, said he hoped they presaged a "new chapter, not only with the three European countries, but with Europe as a whole."

"If the two sides pursue these negotiations in good faith I think the two sides will arrive at a new stage," he told reporters.

But the way ahead is fraught with difficulties since Iran says its suspension of uranium enrichment, a key step in making nuclear fuel, is a temporary measure designed to show its intentions are peaceful.

The European Union negotiators, Britain, France and Germany, want the freeze to become permanent.

In addition, Iran's government spokesman Abdollah Ramazanzadeh said on Monday that Tehran was also sticking to its demand that 20 centrifuges -- the machines used to enrich uranium -- be exempted from its current suspension of enrichment activities and be used for "research".

"The question of halting research is not on the agenda," he said.

The United States charges that Iran is using the Paris agreement to gain time to enable it secretly to develop nuclear weapons, a charge strongly denied by Tehran.