VIENNA (Reuters) - Iran has yet to give an explanation over a small quantity of missing uranium metal from a research site, the U.N. nuclear watchdog said in a report that voiced concern over possible military links to Tehran's nuclear program. The discrepancy in uranium quantities at the research site in the Iranian capital came to light after measurements by international inspectors last year failed to match the amount declared by the laboratory. Experts say the quantity of natural uranium that isn't accounted for is too small to be used for a bomb, but that the metal could be relevant to weapons-linked tests. The United States has expressed concern the material may have been diverted to suspected weapons-related research . U.N. inspectors have sought information from Iran to help explain the issue after their inventory last August of natural uranium metal and process waste at the research facility in Tehran measured 19.8 kg (43.6 pounds) less than the laboratory's count. The 11-page IAEA report also showed that Iran had sharply increased its uranium enrichment drive. The findings, which added to fears of escalating tension between Iran and the West, sent oil prices higher. The report will likely add to Israeli and U.S. fears that the Islamic Republic is pushing ahead with atomic bomb plans. U.S. Secretary of State Hillary Clinton said the report "is concerning, and raises a lot of worrisome questions." "We continue to urge Iran to abide by its international obligations, and that is something that countries everywhere do, and we want to see them do it, and we hope that they will be listening," Clinton told Reuters TV during a visit to Tunisia. Israel has threatened Iran with pre-emptive strikes on its nuclear sites. An Israeli security official who advises on nuclear issues, speaking to Reuters on condition of anonymity pending a formal government response, said the report proved that Iran was avoiding giving answers. "The report clarifies starkly that there has been no progress on the questions dealing with military dimensions, and ... keeps reiterating Iran's violations of IAEA and United Nations Security Council resolutions," the official said. "What we have here, in essence, is a dead end," the Israeli official said. Reporting by Fredrik Dahl, additional reporting by Dan Williams in Jerusalem and Ashraf Fahim in Tunis.; Editing by Ori Lewis and Alessandra Rizzo