But three diplomats from Western nations accredited to the International Atomic Energy Agency said that the starting date was set and that they expected the dispute over the venue to be resolved in time.
The bickering between Iran and the world powers over a site after days of talks appeared to reflect the deep differences that have doomed previous meetings, during which Iran has refused to even discuss international demands that it curb nuclear activities that could be used as part of a weapons program.
The main issue remains uranium enrichment. Iran says the expansion of its enrichment program is meant only to provide nuclear fuel, denies any interest in developing an atomic bomb, and says the right of countries to enrich nuclear power is enshrined in the Nonproliferation Treaty. But the United States and others say Iran's nuclear record causes concern.
The United States and its Western allies have agreed on sanctions designed to add weight to U.N. penalties on Iran because of its enrichment program, while attempting to persuade Israel there is no need now to attack Iran's nuclear facilities.
At a nuclear security summit in Seoul, South Korea, President Obama urged Iran on Monday to heed U.N. Security Council demands on an enrichment freeze.
Regarding the location of the April 13 talks, Iran favors Istanbul, Turkey. But because the last talks there failed, the six powers involved - the United States, Britain, France, Germany, Russia, and China - oppose that venue, said the three diplomats, who spoke on condition of anonymity. Another possible venue, Vienna, is opposed by Iran because it is the home of the IAEA, the U.N. monitor overseeing Tehran's nuclear activities, the diplomats said.