The UN's nuclear watchdog says it has received new information on "possible military dimensions" to Iran's nuclear development programme.
The International Atomic Energy Agency (IAEA) said the report raised "further concerns" about Iran's activities.
It urged Tehran to co-operate fully with its investigations in alleged weapons experiments, saying it had not done since 2008.
Iran insists its nuclear programme is entirely peaceful.
The IAEA report was obtained by the BBC and made available online by the Institute for Science and International Security (Isis).
It says Iran is "not implementing a number of its obligations including clarification of the remaining outstanding issues which give rise to concerns about possible military dimensions to its nuclear programme".
The country was also "not providing the necessary co-operation to enable the Agency to provide credible assurance about the absence of undeclared nuclear material and activities in Iran, and therefore to conclude that all nuclear material in Iran is in peaceful activities".
"Based on the agency's analysis of additional information since August 2008, including new information recently received, there are further concerns which the agency also needs to clarify with Iran," says the report.
Among those concerns were that Iran was not engaging with the IAEA on allegations that it was developing a nuclear payload for its missiles.
Six world powers are negotiating with Iran over its nuclear programme, and the country is subject to United Nations Security Council sanctions over its refusal to halt uranium enrichment.
Enriched uranium can be used for civilian nuclear purposes, but also to build atomic bombs.
The UN has imposed four sets of sanctions on Iran over the years.
While these have made it more difficult for Iran to acquire equipment, technology and finance to support its nuclear activities, they have not stopped trading in oil and gas - the major sources of Iran's income.