VIENNA (AP) -- Iran has rapidly ramped up production of higher-grade enriched uranium over the last few months, the U.N. nuclear agency said Friday, 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.
Instead, it 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 program.
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."
The report was issued to the IAEA's 35-nation board and the U.N. Security Council as the latest update on what the agency knows or suspects about Iran's nuclear program.
It comes amid heightened tensions caused by Iran's refusal to rein in nuclear activities
Instead, it has retaliated by imposing oil embargoes on Britain and France and threatening other European nations that act against it with similar punishment, as well as bluntly warning its foes that it is ready to strike pre-emptively.
Such threats are clearly directed at Israel, which, along with the United States, has not ruled out military attacks if diplomacy fails to halt Iran's nuclear drive. Washington has in recent weeks tamped down the rhetoric as it seeks to defuse tensions, but Israel refuses to follow, saying it alone will decide on what actions to take to protect its security.
In an apparent attempt at damage control before his March trip to the White House, Israeli Prime Minister Benjamin Netanyahu has ordered his ministers not to speak publicly about Iran, Israeli officials said Friday.
Israel views Iran as an existential threat, citing frequent Iranian calls for Israel's destruction, its support for violent anti-Israel militant groups and its nuclear and long range missile programs.
Iran's refusal to scrap its uranium enrichment program is a key worry -- it had already triggered four sets of U.N. Security Council sanctions even before the recent flurry of penalties imposed separately by countries or groups of countries.
Enrichment at low levels provides reactor fuel -- but that can be turned into weapons grade material by gradually re-enriching to levels of 90 percent or more.
Iran has enriched tons of fuel-grade material since its clandestine program was discovered 10 years ago, and worries have been compounded by its decision two years ago to start enriching at a higher level that can be turned into fissile warhead material much more quickly and easily than its low enriched uranium.
Adding up verified figures and Iranian estimates, the report said that Tehran has to date produced about 110 kilograms -- over 240 pounds -- of higher-level uranium enriched to 20 percent. That is about half of what it would need to arm a nuclear warhead.
And it has increased its 20-percent production capability by adding hundreds of centrifuges to a facility dug into a mountain since the last IAEA report in November, said the agency. That structure may be impervious to any bunker-busting bombs Israel has at its disposal.
Beyond that, Iran has added about 2,600 centrifuges producing lower-enriched uranium below 5 percent since that last IAEA report, so that about 9,000 centrifuges are now churning out the lower-enriched material.
The report said that when verified amounts and Iranian estimates are tallied the Islamic Republic has produced nearly 5,500 kilograms -- over 12,000 pounds -- of lower enriched material. Although the lower-enriched material takes longer to convert, that would be enough for an additional four nuclear warheads, should Iran decide to make such weapons.
The agency already reported in November that nearly 20 kilograms -- almost 45 pounds -- of uranium metal were missing from Iran's inventory at a research laboratory. While that amount is too little to be molded into a nuclear warhead, diplomats have said it could be used for related experiments.
On Friday, the IAEA report said that it had requested access to records and personnel involved in the experiments that made the metal but "Iran indicated that it no longer possessed the relevant documentation and that the personnel involved were no longer available."