UNITED NATIONS - Iran is believed to be withholding information on studies it allegedly conducted as part of a secret project to develop nuclear weapons, a watchdog agency said Monday. Tehran's nuclear plans are a matter of serious concern and require "substantial explanations from Iranian officials," the International Atomic Energy Agency said in a report, The New York Times reported. The report said Iranian officials have not cooperated with an investigation into whether its nuclear ambitions were for peaceful purposes, as the government of President Mahmoud Ahmadinejad has said. The nine-page IAEA report says Western intelligence agencies have documents that indicated the Iranians have ventured into explosives, uranium processing and a missile warhead design, which are ordinarily associated with with building nuclear arms. The documents cited by the Western intelligence agencies to make their case against Iran are forged or fabricated, Iran says. Ali Asghar Soltanieh, Iran's ambassador to the IAEA, said the report vindicated Iran's nuclear activities. He said another document in the report shows Iran's entire nuclear activities are peaceful, the Fars News Agency quoted him as saying.