ISTANBUL—Six world powers emerged from the first nuclear talks with Iran in over a year on a positive note Saturday.

“The discussion on the Iranian nuclear issue has been constructive and useful,” said European Union Foreign Policy Chief Catherine Ashton, the lead negotiator. “We want now to move to a sustained process of serious dialogue, where we can take urgent, practical steps to build confidence.”

After a two-and-a-half-hour morning plenary session, officials agreed to hold another meeting in Baghdad May 23, where they hope to devise a concrete work plan to address urgent concerns about Iran's advancing nuclear program. Following that session, Iran held bilateral meetings with Russia, China and Ashton, but declined to meet one-on-one with American diplomats.

"It's quite an encouraging beginning of this negotiating process," said Greg Thielmann, a former U.S. official with the Arms Control Association, in a telephone interview Saturday. "Our expectations were modest, just because the history of this difficult relationship between the United States and Iran in particular. I think we have to be satisfied from all indications so far that a serious negotiating process has begun."