The UN nuclear watchdog fears Iran may be working now to develop a nuclear payload for a missile, the agency said in a confidential report on Thursday obtained by Reuters.
The International Atomic Energy Agency report also confirmed Iran had produced its first, small batch of uranium enriched to a higher purity - 20% - but said the Islamic Republic had failed to give inspectors the required advance notice.
Both developments will stoke Western concerns that Iran is secretly bent on developing nuclear weapons capacity from the enrichment process. Tehran says the effort is meant only to yield electricity or radio-isotopes for agriculture or medicine.
The IAEA has been investigating for several years Western intelligence reports indicating Iran has coordinated efforts to process uranium, test explosives at high altitude d revamp a ballistic missile cone in a way suitable for a nuclear warhead.
In 2007 the United States issued an assessment saying Iran had halted such research in 2003 and probably not resumed it.
But its key Western allies believe Iran continued the program - and the IAEA report offered independent support for that perception for the first time.
"The information available to the agency is extensive ... broadly consistent and credible in terms of the technical detail, the time frame in which the activities were conducted and the people and organizations involved," the report said.
"Altogether this raises concerns about the possible existence in Iran of past or current undisclosed activities related to the development of a nuclear payload for a missile."
It was unusually blunt language in the first IAEA report on Iran under new Director-General Yukiya Amano, who is seen as more inclined to confront Iran over its behavior than his predecessor, Mohamed ElBaradei.
The report, to be considered at a March 1-5 meeting of the IAEA's 35-nation governing body, said it became harder to unearth relevant information as time passed, and it was therefore vital for Iran to cooperate with IAEA investigators "without further delay."