Iran has rapidly ramped up production of higher-grade enriched uranium over the last four months, the UN nuclear agency says in a confidential report.
The report fuels fears about how quickly the Islamic Republic could produce an atomic bomb.
The International Atomic Energy Agency also said Iran had failed to give a convincing explanation about a quantity of missing uranium metal.
Diplomats have said the missing amount could be used for experiments used to arm a warhead.
Iran insists it is not interested in nuclear weapons and says all of its activities are meant either to generate energy or to be used for research.
But the report expresses “serious concerns regarding possible military dimensions to Iran’s nuclear programme.”
The report also confirmed that two IAEA missions to Tehran within less than a month had failed to dent Iran’s refusal to assist an IAEA probe of suspicions that the country has been secretly working on aspects of a nuclear weapons programme.
The confidential report obtained by The Associated Press said the agency continues to have “serious concerns regarding possible military dimensions to Iran’s nuclear programme”.
The report was issued to the IAEA’s 35-nation board and the UN Security Council as the latest update on what the agency knows and what it suspects about Iran’s nuclear programme.
It comes amid heightened tensions caused by Iran’s refusal to reign in nuclear activities that much of the world fears could be redirected toward a weapons programme.
A rapid series of sanctions imposed by the US, the European Union and others imposed on Tehran have only increased acrimony without any sign that Iran is ready to compromise.
Instead, it has retaliated by imposing oil embargoes on Britain and France and threatening other European nations that act against it with similar punishment.