Tehran says it needs enriched uranium for nuclear fuel but the U.S. and other nations fear that could be used to make weapons. The U.S. says Iran is one to six years away from being able to do so.
Iran had signaled earlier that it might not meet Western demands for a deal under which it would ship most of its enriched material out of the country.
Another problem appeared to be Iranian insistence that France be excluded from any participation in plans to turn the enriched material into fuel for Tehran's research reactor, the diplomats told The Associated Press.
With the main talks stalled,the U.S., Russia, France and Iran staged separate meetings. Among these was an early evening encounter between the Iranian and American delegation "under the auspices" of ElBaradei, said Mike Hammer, spokesman for the U.S. National Security Council. He gave no details on the substance of the talks.
Iran and the U.S. held their first direct talks in nearly three decades earlier this month in Geneva, where the five U.N. Security Council members and Germany met with Tehran to try to launch substantive negotiations meant to persuade Tehran to freeze its enrichment program.
ElBaradei was cautiously upbeat with reporters shortly after the talks convened.
"We are making progress although slower than expected," he said. "But we are moving forward ... we hope to be able to reach an agreement."
ElBaradei spoke of a "question of confidence building and guarantees" - a possible allusion to the direct discussions between the Iranians and Americans and the need by both sides to defuse decades of distrust.