The UN atomic agency revealed that Iran received black-market designs to encase weapons-grade uranium, and diplomats said they appeared to be part of blueprints for a nuclear warhead. A senior US diplomat called the find disturbing and other diplomats accredited to the International Atomic Energy Agency said they expected the United States and its allies to use it in their push to have Teheran referred to the UN Security Council as early as next week. "You've given the world cause for concern," US State Department spokesman Adam Ereli said in Washington. "The international community doesn't like what it sees." The revelations came Friday as Iran said it had begun converting a second batch of uranium into gas, a step that brings it closer to producing the enriched uranium used to either generate electricity or build bombs. The European Union, with US support, has been calling on Iran to reimpose a freeze on conversion since August. But the nation's top nuclear negotiator, Ali Larijani, told state TV the country had started converting a second batch of uranium. "This job is done and the plant is continuing its activity," Larijani said in the interview recorded late Thursday and broadcast Friday. The IAEA said Friday that Iran received the detailed designs from the network run by Abdul Qadeer Khan, the father of Pakistan's nuclear program. His network supplied Libya with information for its now-dismantled nuclear weapons program that included an engineer's drawing of an atomic bomb. The document given to Iran in 1987 showed how to cast "enriched, natural and depleted uranium metal into hemispherical forms," said a confidential IAEA report. IAEA officials refused to comment on the implications of the finding. But diplomats close to the agency said it appeared to be a design for the core of a nuclear warhead. The report said Iran insisted it had not asked for the designs but was given them anyway by members of the nuclear network - something a senior official close to the agency said the IAEA was still investigating. The diplomats requested anonymity in exchange for discussing the confidential report obtained by The Associated Press. The document was prepared for Thursday's meeting of the IAEA's 35-nation board, which could decide to refer Teheran to the UN Security Council for possible sanctions for violating an international nuclear arms control treaty. Most board nations are concerned that Iran has resumed uranium conversion - a precursor to enrichment - and has refused to meet all IAEA requests about a nuclear program that was clandestine for nearly 20 years until discovered three years ago. The United States insists Iran is trying to make nuclear weapons, while Iran maintains its program is strictly for generating electricity. The chief US delegate to the IAEA, Gregory L. Schulte, said Washington was "very concerned" about the design, along with the "large cache of documents uncovered by the agency" showing detailed instructions on how to set up uranium enrichment facilities. "This opens new concerns about weaponization that Iran has failed to address," he told reporters. Former nuclear inspector David Albright said the design is "part of what you need ... to build a nuclear weapon." Although it's not a "smoking gun" proving Iran was secretly developing nuclear weapons, the find casts doubt on previous Iranian assertions it had no documents on making such arms, said Albright, now the head of the Institute for Science and International Security in Washington. The report said Iran had handed over black-market documents revealing detailed instructions on setting up the complicated process of uranium enrichment. Khan has acknowledged selling secrets to Iran, Libya and North Korea. The report also suggested Iran had something to hide, saying it continues to refuse access to a sensitive site where it could be storing equipment that could help investigators determine whether the military is running a secret nuclear program. It said more transparency by Teheran was "indispensable and overdue" as agency inspectors try to determine if Iran's military secretly ran its own nuclear program parallel to a civilian one.