VIENNA — Iran has rapidly ramped up production of higher-grade enriched uranium over the past four months, the U.N. nuclear agency said today in a confidential report that feeds concerns about how quickly the Islamic republic could produce an atomic bomb.

The report by the International Atomic Energy Agency also said Iran had failed to give a convincing explanation about a quantity of missing uranium metal. Diplomats say the amount unaccounted for is large enough to be used for experiments in arming a nuclear missile.

Iran insists it is not interested in nuclear weapons and says its activities are meant either to generate energy or to be used for research. But the report contained little assurances the country’s activities are purely peaceful.

The confidential report obtained by The Associated Press said the agency continues to have “serious concerns regarding possible military dimensions to Iran’s nuclear program.”