Iran's top nuclear negotiator Saeed Jalili said on Thursday that nuclear talks with the EU'S foreign policy chief Catherine Ashton were "long and useful" and that both sides have decided to continue the discussions.

"Last night, as Lady Ashton said, we had long, useful talks," Jalili said in Istanbul. "We had the chance to go into details. We decided to continue working and keep on our talks."

Ashton called the talks "useful" in a statement on Wednesday and said it was time to "reflect on how to go to the next stage of the process".

The two top negotiators were meeting for the first time since fruitless discussions between Iran and six major powers -- the US, China, Russia, Britain, France and Germany -- in April over Tehran's controversial nuclear ambitions.

That round in Almaty, Kazakhstan, had ended with Ashton saying the two sides remained "far apart" despite the six powers known as P5+1 having sweetened an earlier offer.

But Jalili said Thursday that "our proposals in Almaty were very good."

He pointed out that the world powers had said they wanted to negotiate within the group before responding to Tehran's offers.

"We hope they will agree to those proposals. We hope they will turn our proposal for cooperation into an opportunity," Jalili said, without elaborating.

Jalili said Iran was "ready to continue the talks any time" but insisted that Tehran's right to peaceful uranium enrichment "should be recognised".

Enriched uranium is at the heart of the international community's concerns because it can be used to make a nuclear bomb, as well as for energy production.

The UN Security Council has passed multiple resolutions calling on Iran to suspend all uranium enrichment, imposing several rounds of sanctions on the Islamic republic.

Additional US and EU sanctions last year began to cause major economic problems by targeting the Persian Gulf country's vital oil sector and financial system.

Jalili said the world powers were proposing a "step by step" approach in the nuclear talks.

But he noted that Tehran wants any concessions it makes to be reciprocated by the West, particularly with regards to easing sanctions.

"We discussed last night how we can place the process on a framework of reciprocity," he said.

"Some opportunities are good to use when they come by, otherwise they may be lost," he warned.

A separate set of talks held Wednesday in Vienna between Iranian officials and the UN atomic agency ended without a deal.

Efforts to resolve the long-running dispute are complicated by the fact that Iran goes to the polls on June 14 to choose a successor to President Mahmoud Ahmadinejad, with Jalili himself among the hopefuls.