Nuclear talks between Iran and the UN atomic agency failed yet again Wednesday, as the top US diplomat in separate six-party negotiations warned Washington's patience was wearing thin.

International discussions on Iran's nuclear ambitions were held in Istanbul and Vienna but the Western reactions afterwards ranged from tepid to disappointed.

While the EU's foreign policy chief met Iran's top nuclear negotiator in Istanbul for the first time since a fruitless effort in early April, the UN atomic body's chief inspector admitted the parallel Vienna talks had ended without a deal.

"We could not finalise the structured approach document that has been under negotiation for a year and a half," the International Atomic Energy Agency's (IAEA) chief inspector Herman Nackaerts told reporters.

"Our best efforts have not been successful so far."

Iran's envoy to the IAEA, Ali Asghar Soltanieh, was characteristically more upbeat, saying that the next meeting would see a deal finalised, although Nackaerts said no new gathering had been arranged.

"The aim of this ... is to bridge the gaps towards a conclusion of the text by the next meeting," Soltanieh told a joint press briefing.

EU foreign policy chief Catherine Ashton called her discussions with Iran's chief negotiatior Saeed Jalili "useful".

"We will now reflect on how to go on to the next stage of the process," she said in a statement.

The IAEA wants Iran to grant access to sites, documents and scientists involved in Tehran's alleged efforts to develop atomic weapons, which the agency suspects mostly took place before 2003 but are possibly still ongoing.

Iran says the IAEA's findings are based on faulty intelligence from foreign spy agencies such as the CIA and Israel's Mossad -- intelligence it complains it has not even been allowed to see.

Nine rounds of talks since the publication of a major IAEA report in November 2011 have produced no breakthrough.

Wendy Sherman, the head of the US delegation in parallel six-party talks with Iran, meanwhile indicated growing frustration in Washington over the lack of progress, and not just between Iran and the IAEA.

"At some point, the director general of the IAEA will have to return to the (UN) Security Council and say 'I can go no further; there has been no response; you have to take further action'," she told the Senate Foreign Relations committee.

Sherman also said that in parallel diplomatic efforts between Iran and the six major powers -- the US, China, Russia, Britain, France and Germany -- Tehran was "putting very little on the table and asking a lot in return."

The six countries "have stood united in putting what we think is a very reasonable and balanced confidence-building ... proposal in front of Iran," she said.

The talks between Iran and five members of the UN Security Council plus Germany -- known as the P5+1 -- are focused more on Iran's current activities, most notably uranium enrichment.

Enriched uranium is at the heart of the international community's concerns since it can be used not only for peaceful purposes such as power generation but also -- when highly purified -- in a nuclear bomb.

The latest P5+1 round in Almaty, Kazakhstan, last month ended with lead negotiator Ashton saying the two sides remained "far apart" despite the P5+1 having sweetened an earlier offer.

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.

A senior US Treasury official, David Cohen, also said Wednesday that the United States was aiming to block gold sales to Iranians to boost pressure on the already embattled rial currency.

Israel, the Middle East's sole if undeclared nuclear-armed state, meanwhile has refused to rule out military action on Iran -- as has US President Barack Obama.

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.