GENEVA  Iran agreed on Thursday in talks with the United States and other major powers to open its newly revealed uranium enrichment plant near Qum to international inspection in the next two weeks and to send most of its openly declared enriched uranium outside Iran to be turned into fuel for a small reactor that produces medical isotopes, senior American and other Western officials said.

Iran’s agreement in principle to export most of its enriched uranium for processing  if it happens  would represent a major accomplishment for the West, reducing Iran’s ability to make a nuclear weapon quickly and buying more time for negotiations to bear fruit.

If Iran has secret stockpiles of enriched uranium, however, the accomplishment would be hollow, a senior American official conceded.

The officials described the long day of talks here with Iran, the first such discussions in which the United States has participated fully, as a modest success on a long and complicated road. Iran had at least finally engaged with the big powers on its nuclear program after more than a year and had agreed to some tangible, confidence-building steps before another meeting with the same participants before the end of this month.