The Obama administration pressed Iran to account for a discrepancy of nearly 20 kilograms in its reporting to the United Nations' nuclear agency on how much natural uranium metal it has in its stockpile. U.S. diplomats told a quarterly gathering Friday of the International Atomic Energy Agency's 35-nation board that Washington fears this nuclear material could be diverted for use in Tehran's weapons programs. Natural uranium metal can specifically be utilized as a surrogate material to conduct simulated tests of nuclear detonations, according to U.S. officials. It can also be used to produce high-explosive weapons, such as armor-piercing rockets. The IAEA calculated 20 kilos more than Iran had reported. "It remains to be seen whether this discrepancy could ultimately represent another piece in the puzzle the IAEA is assembling to show Iran's nuclear weapons-related activities," said Glyn Davies, U.S. ambassador to the IAEA. "The secretariat and the board should accept no delay in Iran's response." The agency's board voted by consensus Friday to formally censure Tehran for its nuclear activities, but didn't take steps to initiate new economic sanctions on Iran. The resolution followed the release last week of an IAEA report that detailed Tehran's alleged efforts to develop the technologies used to make nuclear weapons. The study concluded that Iran has worked to develop nuclear-tipped midrange missiles and bomb-triggering systems. It also cited the discrepancy between Iran's reporting of its natural uranium metal stockpile and what IAEA inspectors have verified. "This in many ways could prove the most direct link between Iran's military activities and its stockpile of nuclear materials," said an American official. To increase pressure on Iran, the Obama administration is set to announce a new round of bilateral sanctions on Tehran early next week, according to people briefed on the moves. The sanctions will seek to further diminish Iran's ability to refine oil into petroleum products and to conduct dollar-denominated transactions. The U.S. isn't expected to directly sanction Iran's central bank, as many on Capitol Hill have demanded. Iran is the third-largest exporter of oil among the Organization of Petroleum Exporting Countries. However, it is forced to important roughly 40% of its gasoline due to lack of refining capability. Iranian officials on Friday rejected all the IAEA's allegations, saying they were politicized and based on falsified information. They stated that none of Tehran's nuclear materials have ever been diverted for military purposes. Tehran's ambassador to the IAEA, Ali Asghar Soltanieh, also indicated that Iran may limit its cooperation with the U.N. agency in the future. He rebuffed a call this week from IAEA Director General Yukiya Amano for a high-level delegation to visit Iran in coming weeks to discuss the report's conclusions. And he said Iran would boycott an IAEA conference next week that will focus on establishing a nuclear weapons-free zone in the Middle East. Most Arab states and Israel have confirmed their attendance. "We will not suspend our [uranium] enrichment activities or work even for a second," Mr. Soltanieh told reporters in Vienna. "We are even more determined." Nuclear experts said the IAEA has differed with Iran before about the amounts of nuclear materials it possesses. It some cases, they said, Tehran has even overestimated the amount of enriched uranium it has produced. But the agency and Iran have in the past worked to resolve their differences. The IAEA's report said the agency believes Iran has conducted some simulated tests of nuclear detonations. Natural uranium metals, these experts said, have many of the same properties as the highly enriched uranium used in a nuclear bomb, but without the fissile reaction. U.S. officials tried to play down the charges by some American politicians that the new IAEA resolution was too soft on Iran. A large number of Democratic and Republican lawmakers have been pressing the White House to unilaterally sanction Iran's central bank, the principal conduit for the country's oil sales. "I think this resolution does the trick," Mr. Davies told reporters. "It sends the right signal to Iran."