ISTANBUL — In a 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 over Tehran’s 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.

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.

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.

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.