BRUSSELS—Talks between Iran and six major powers on its nuclear program look set to start on April 14 in Istanbul, a senior European Union diplomat said.

The so-called P5+1—which groups together the U.S., Russia, China, Germany, the U.K. and France—last met with Iran in January 2011, also in Istanbul.

Those talks broke down after just a couple of days, and have been followed by a growing series of U.S. and European Union sanctions intended to choke off Iran´s oil revenue and press it back to negotiations.

As part of the talks, the Obama administration and its diplomatic partners are preparing to ask Tehran to cease enriching uranium at levels dangerously close to weapons grade, said officials briefed on the plan. The U.S. and many others say the Iranian program is aimed at developing nuclear weapons. Tehran denies that charge.

EU foreign-policy chief Catherine Ashton, who heads the P5+1 group, has said the talks need time. "If this is successful, it will be a sustained process," she said at a recent Brussels forum.

Mrs. Ashton has been a defender of the track approach that by intensifying sanctions, Iran could be pressured into meaningful talks. President Barack Obama recently said the window for a diplomatic solution with Iran is narrowing.

Iran didn´t want to meet in any country that placed sanctions on it, according to a person familiar with the discussions.

—Jay Solomon contributed to this article. Write to Laurence Norman at laurence.norman@dowjones.com (Copyright © Dow Jones & Company, Inc.) 03/28/12)