MOSCOW - The first day of the latest round of nuclear negotiations with Iran ended without any breakthrough in what was described as a "tough and intensive" day of talks.

"It wasn't easy," Michael Mann, a spokesman for European Union High Representative Catherine Ashton, told reporters. He added that there are still disagreements on key issues.

A Western diplomat described the first day of talks as more of an exchange of views and less of a negotiation. Both sides agreed to meet again on Tuesday.

The high stakes meeting in Moscow between a group of world powers and Iran came at a critical and sensitive time. Tough U.S. and European 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 comes at a sensitive time for President Obama who, in a tight election year, must balance taking a tough stance on Iran, as his presumptive Republican opponent 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. Officials were cautious about promising too much, but were hopeful that Iran would begin to negotiate the substance of a proposal by the group, which includes the five permanent members of the U.N. Security Council plus Germany.

In that area there was some progress, as the Iranian delegation showed a lengthy PowerPoint presentation that addressed several aspects of the proposal. However, a Western diplomat close to the talks, who requested anonymity in order to discuss sensitive diplomatic talks, said the Iranians also deviated into discussing what were described as "old grievances."

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 20 percent, levels that many fear could help them perfect techniques to produce bomb-grade fuel. The proposal also calls on Iran to ship that fuel out of the country and cease activities at an underground nuclear site at Fordow, near the city of 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.

"Iran should come prepared to negotiate seriously and take concrete steps to address the unified proposal laid out by the E3+3 in Baghdad addressing all aspects of 20 percent enriched uranium, including activities at Fordow, and enrichment and stockpiling of 20 percent uranium throughout Iran," another Western official said before talks began, referring to the negotiating group by one of its acronyms.

"The international community for its part is poised to take reciprocal steps in exchange for verifiable Iranian actions to address these concerns," the Western official added.

Those steps could include the easing of sanctions, providing Iran with 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. Western officials were skeptical about accepting that demand.

During Monday's meeting in Moscow the Iranians again expressed their desire to see sanctions lifted and that the international community recognize Iran's right to to enrich. A Western diplomat said they offered to formalize a fatwah, or Islamic legal ruling, in a United Nations resolution that would state that Iran has no intention of producing nuclear weapons.

While sensitive to the weight of a fatwah, the diplomat was skeptical such guarantees would be enough to appease international concerns about the nature of Iran's nuclear program.

Another 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.

"If Iran remains unwilling to take the opportunities these talks present, it will face continuing and intensified pressure and isolation," the official said.

Iran apparently showed just enough willingness to talk at the Baghdad meeting that the group of world powers agreed to hold Monday's meeting.

The official said there was a "fairly narrow" common ground with Iran, but on key issues like confirmation that Iran's enrichment of uranium up to 20 percent was on the table.

The talks are slated to take place for two days, but officials say they are prepared to stay longer if progress is being made.

However, officials warned that international patience with Iran is not unlimited.