MOSCOW - Any hopes of a swift diplomatic conclusion to the standoff over Iran's nuclear program were dashed as the latest round of high stakes negotiations ended today with only a vague plan for future negotiations.

Catherine Ashton, the European Union's top foreign policy official who has been leading negotiations for six world powers, described the talks as "tough and frank."

"It remains clear that there are significant gaps between the substance of the two positions," she told reporters after a long day of talks in a Moscow hotel.

The two sides agreed to hold a lower level meeting among technical experts in Istanbul on July 3, but made no concrete plans for another high level meeting.

The Iranians said they rejected the initial proposal for a meeting on July 2 because they say that was the anniversary of the United States shooting down of an Iranian airliner. The U.S. side denied this mix-up was a moment of real tension during the talks.

Following the July 3 meeting, deputy negotiators from both sides will meet and, depending on the outcome, top rank negotiators could follow up with a phone call to decide whether to hold another full meeting.

Iran's lead negotiator Dr Saeed Jalili told reporters the Moscow talks were "more serious, more realistic" than previous rounds.

"On this path there is no reason or excuse to have doubts regarding the peaceful" nature of Iran's nuclear program, he said through an interpreter.

Jalili reiterated the Iranian position that it has the "inalienable right" to enrich uranium, something the world powers have been reluctant to recognize.

Ashton noted that this was the first time Iran had responded to the substance of a proposal put on the table at the last meeting in Baghdad. That plan called on Iran to end its enrichment of uranium at 20 percent, shutter its underground Fordow nuclear facility, and ship its stockpile of 20 percent uranium out of the country. In exchange, sanctions on Iran could be eased and Iran could receive badly needed spare parts for its aging civilian aircraft. It could also receive medical isotopes for treatment of cancer patients.

Western officials, however, said that the Iranian response, delivered in a lengthy PowerPoint presentation on Monday afternoon, also rehashed what were described as "old grievances" and raised new questions about the proposal.

Tehran insists its program is peaceful, but the United States and its allies are not convinced, fearing it is a cover for plans to develop nuclear weapons.

The talks, which include the five permanent members of the U.N. Security Council and Germany, came at a critical and sensitive time. U.S. oil sanctions on Iran will go into effect within weeks, hitting Tehran in its most lucrative sector and increasing pressure.

The U.S. and other allies have also been trying to hold off Israel, which has shown impatience with diplomatic efforts to prevent Iran from obtaining a nuclear weapon, and has warned of a pre-emptive military strike.

And it came at a sensitive time for President Obama who, in a tight election year, must balance taking a tough stance on Iran, as Mitt Romney has done, and the danger of upsetting oil markets that could derail economic recovery.

This was the third meeting with Iran this year. At the last meeting in Baghdad, the U.S. and its allies put forth a proposal under which Tehran would end its efforts to enrich uranium at levels that many fear could help them perfect techniques for producing bomb-grade fuel, as well as ceasing activities at an underground nuclear site near Qom that might be impervious to attack. Iran insists its nuclear program is for civilian purposes, but many in the international community fear it is a pretense for creating a nuclear weapon.

In exchange, the six countries dangled offers of much needed spare parts for its aging civilian aircraft, providing Iran with fuel for a research reactor in Tehran and the provision of medical isotopes.

Iran, meanwhile, voiced its own broad demands during the Baghdad meeting. The key demand, according to recent reports, is recognition of Iran's right to enrich uranium.

A Western official credited recent sanctions with pressuring Iran to the negotiating table and hinted that countries were prepared to take additional punitive steps if Iran fails to comply.

Officials also warned that international patience with Iran is not unlimited.