Iran has denied it intends to make a nuclear weapon
The UN nuclear watchdog has expressed concern that Iran may currently be trying to develop a nuclear payload for a missile.
The information is carried in a leaked confidential report by the Vienna-based International Atomic Energy Agency.
It also confirmed that Iran had begun enriching uranium to higher levels.
Iran has denied it is trying to build a nuclear weapon but the US and other nations have been pressing to extend UN sanctions on the issue.
This is the first IAEA report on Iran compiled under its new director-general, Yukiya Amano, and will be put to the governing body at a 1-5 March meeting.
The White House said the findings showed Iran was failing to meet its international responsibilities and would have to face the consequences.
The BBC's Bethany Bell, in Vienna, says the confidential report is likely to intensify the row over Iran's controversial nuclear programme.
She says this appears to be the first time the IAEA has questioned the current as well as past activities in Iran that opponents fear could be related to the development of a nuclear missile.
Iran says it is increasing uranium enrichment from 3.5% needed for commercial nuclear reactors
Iran says it has started enriching to 20%, needed for a research reactor near Tehran
Weapons-grade uranium is at least 90% enriched
Experts say achieving 20% is a key step towards weapons grade
The report says its information was "consistent and credible in terms of the technical detail, the timeframe in which the activities were conducted and the people and organisations involved".
It added: "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."
The report says it is vital Iran co-operates with IAEA investigators "without further delay" as its resistance added to concerns "about possible military dimensions to Iran's nuclear programme".
The report also confirmed Iran had produced a small batch of uranium enriched to the higher 20% level.
Until now, Iran has produced uranium enriched to 3.5% for power stations.
It says it needs the more highly enriched uranium for a reactor producing medical isotopes.
Western powers fear Iran is heading towards enriching uranium to 90% - to produce a nuclear weapon.
US state department spokesman PJ Crowley said the findings reflected "our ongoing concerns about Iran's activities".
Iran's envoy to the IAEA, Ali Asghar Soltanieh, told the Irna news agency the report "verified the peaceful, non-military nature of Iran's nuclear activities".
The UN Security Council has already imposed three rounds of sanctions on Iran in an effort to make it cease all uranium enrichment activities and heavy water projects, which could produce weapons-grade plutonium.
Last October it was thought a deal had been struck to allow Iran to send about 70% of its low-enriched uranium to Russia and France, where it would be processed into fuel for a research reactor.
After many delays Iran told the IAEA in January that it did not accept the terms of the deal and has pressed on with enrichment.
The US has since pushed for a new round of "strong" sanctions and has the support of the UK and France. Russia has urged Iran to abide by its earlier agreements.
The other permanent UN Security Council member, China, would still have to be persuaded not to stand in the way of a fourth round of sanctions.