(CNN) -- The International Atomic Energy Agency issued a critical report Tuesday saying that it has "serious concerns" about Iran's nuclear program and has obtained "credible" information that the Islamic republic may be developing nuclear weapons.





The IAEA report, the most detailed to date on the Iranian program's military scope, found no evidence that Iran has made a strategic decision to actually build a bomb. But its nuclear program is more ambitious and structured, and more progress has been made than previously known.





"The agency has serious concerns regarding possible military dimensions to Iran's nuclear program," the report said. "After assessing carefully and critically the extensive information available to it, the agency finds the information to be, overall, credible. The information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device."





U.S. State Department spokeswoman Victoria Nuland said the report had just arrived and refrained from commenting on details at an afternoon briefing. But a senior U.S. official called the report "a big deal."





"The report is very comprehensive, credible, quite damning, and alarming," the official said.





Read the IAEA report here





Iranian President Mahmoud Ahmadinejad slammed the report as a fabrication of facts aimed at satisfying U.S. allegations about Iran's nuclear program.





Ahmadinejad essentially called Yukiya Amano, the director general of the IAEA, a U.S. puppet and said the United Nations agency has no jurisdiction in Iran.





"The Americans have fabricated a stack of papers and he keeps speaking about them," he said on state-run Press TV. "Why don't you do a report on the U.S. nuclear program and its allies? Present a report on the thousands of U.S. military bases where Washington has nuclear arms that threaten global security."





The IAEA had released another report on Iran in September but this one was highly anticipated because of the military aspect.





Since 2002, the IAEA has regularly received new information pertaining to the development of a nuclear payload for a missile, the report said.





It said Iran has made "efforts, some successful, to procure nuclear related and dual use equipment and materials by military related individuals and entities" and has acquired nuclear weapons information from "a clandestine nuclear supply network."





It has also worked on mastering the design of a nuclear weapon and tested components, the report said.





The IAEA said the some of the activities have both civilian and military applications, but others are specific to nuclear weapons.





Iran has repeatedly insisted its nuclear program is for peaceful, civilian energy purposes only.





According to the IAEA report, Iran is believed to have continued weapons research and technology development after 2003, when the intelligence community thought Iran had stopped. Instead of halting, it seems Iran took a temporary hiatus at the time, although the program progressed at a more modest pace since then, the report said.





After the report's release, top Republicans in Congress called on President Barack Obama's administration to ratchet up economic sanctions against Iran. Rep. Mike Rogers, the chairman of the House Intelligence Committee, raised the fear that Israel would attack Iranian nuclear facilities without further steps by the United States and its allies.





Rogers, R-Michigan, said new sanctions should cut off the supply of refined fuel to Iran and target its central bank, which he said is being used to finance Tehran's nuclear program.





"If we talk about it for a long time, if we're not really leading on it, I will tell you we leave this option to the Israelis," he told CNN's "John King USA."





And in a written statement, Rep. Ileana Ros-Lehtinen called on Congress to pass two bills targeting Iran's energy sector. The Florida Republican leads the House Foreign Affairs Committee, which recently sent legislation to the House floor to do just that.





"If fully implemented, they have the potential to cripple the regime's ability to continue its nuclear program," she said. "If the Iranian regime acquires nuclear weapons capabilities, the U.S., Israel and our other allies in
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