In a rare show of unity, Iran and the world's big powers on Saturday hailed their first nuclear meeting in more than a year as a key step toward further negotiations meant to ease international fears that Tehran may weaponize its nuclear program.

The one concrete reflection of progress was an agreement to meet again on May 23 in Baghdad, a venue put forward by Iran.

But huge hurdles still lie in the way of a common understanding of what Iran should do to end suspicions of its nuclear activities. Those barriers may prove insurmountable considering the differences between Tehran and the six nations trying to persuade it to compromise on its nuclear efforts.

Since revelations surfaced 10 years ago that it was secretly building a uranium enrichment program, Tehran has argued it has a right to enrichment to create reactor fuel under the Nuclear Nonproliferation Treaty, and insisted it will never use that ability to create the fissile core of a nuclear warhead.

But the United States and other countries accuse Iran of repeatedly violating the treaty, and Tehran continues to expand enrichment despite four sets of U.N. Security Council resolutions and other penalties imposed by the U.S., Europe and others. Adding to concerns, it now is enriching uranium to levels closer to the grade needed for nuclear weapons in an underground bunker that could be impervious to attack.

The talks in Istanbul on Saturday saw the United States, Russia, China, Britain, France and Germany sitting at the same table with Iran. Knowing the road ahead is tough, both sides focused on what they said was the positive tone of the talks, in contrast to the previous round 14 months ago.

That last session broke up with no progress after Iranian negotiators refused to even consider discussing enrichment

EU foreign policy chief Catherine Ashton, who formally led the talks on behalf of the six powers, called the meeting "constructive and useful."

She expressed the hope they will lead to "a sustained process of serious dialogue, where we can take urgent practical steps to build confidence and lead on to compliance by Iran with all its international obligations."

Iranian chief negotiator Saeed Jalili said the talks made "some progress." But he acknowledged "some points of difference."

"What we saw today in the talks was the interest of the other party in the talks and cooperation, which is considered positive," he told reporters.

In London, British Foreign Secretary William Hague said the talks were "the first steps" toward the six-nation push to find "a peaceful, negotiated solution to the (Iran) nuclear issue."

"Today's talks were a first step towards that objective, but there is still a long way to go."

Both Jalili and Ashton said there was agreement to move slowly and be guided by reciprocity — meaning that Iran stood to benefit from easing fears about its enrichment program by unspecified rewards from the other side.

Iran hopes those rewards could include easing or delaying sanctions that target its main cash cow, its oil sales. Jalili acknowledged Saturday that Iran would like to avoid those penalties.

"The lifting sanctions is one of the demands by Iranian nation," Jalili told reporters.

But a senior U.S. administration official who demanded anonymity in exchange for discussing strategy at upcoming talks said that was not on the table in the near future.

"One only expects to look at the issue when there are sufficient concrete steps taken" by Iran, she said at a post-negotiation briefing. "Dialogue is not sufficient for any sanctions relief."

Beyond the bite of sanctions, Iran is under threat of Israeli and possibly U.S. military attack unless it makes headway in persuading the international community it is not pursuing nuclear weapons.

The U.S official said Iran's acceptance of the need to discuss its nuclear program appeared dictated by recognition that the diplomatic "window of opportunity was closing" and that the threat of military action potentially growing.

Ashton said there was agreement by both sides that the talks should be guided by the Nonproliferation Treaty, but because Iran says it has never violated that treaty that understanding could prove to be a huge stumbling block to progress.

Top level meetings of the International Atomic Energy Agency, which tries to monitor Iran's nuclear activities, are often dominated by inconclusive debate between Iran and its critics on whether Tehran is in compliance or has broken treaty provisions.

"Under the NPT, the right of enrichment exists for all member countries," Jalili told reporters after the talks, suggesting his country would press that point at follow-up meetings. Ashton, in turn, told reporters that the six seek "to ensure all the obligations under the NPT are met by Iran while fully respecting Iran's right to the peaceful use of nuclear energy."

In its claim to comply with all NPT obligations, Iran asserts that it declares all its nuclear material and allows inspectors to monitor all nuclear facilities.

But IAEA chief Yukiya Amano has said repeatedly that because Iran does not cooperate fully with his agency it cannot guarantee that it is not hiding undeclared nuclear material that could be used for weapons. Additionally, he has spoken of compelling evidence that Iran may have worked on nuclear arms — charges Tehran dismisses as fabrications spread by the United States and Israel.

Officially, the international community's long-term goal remains what it was when nuclear negotiations began eight years ago — persuading Tehran to stop all uranium enrichment and thereby relieve fears that it will use that program to create fissile warhead material.

A senior diplomat involved in the talks said, however, that influential Western nations now are increasingly coming around to the idea that Iran should be allowed to keep some enrichment activity "under the right circumstances," sometime in the future, if all fears about possible Iranian plans to make nuclear weapons are put to rest. He demanded anonymity because his information was confidential.