The International Atomic Energy Agency (IAEA) is "increasingly concerned" about possible activity in Iran to develop a nuclear payload for a missile, it said.
The UN nuclear agency said in a confidential report obtained by Reuters that it continued to receive new information adding to such worries.
The IAEA's information had come from many states and also through its own efforts, and was "broadly consistent and credible in terms of technical detail, the time frame in which the activities were conducted and the people and organizations involved".
The developments highlighted in the IAEA's latest quarterly inspection report are likely to fan Western suspicions about the underlying nature of Iran's nuclear activity, which Western powers suspect is aimed at developing atom bombs.
It could provide additional arguments for the United States and its European allies to further tighten sanctions pressure on Iran, one of the world's largest oil producers.
The IAEA used "stark language" to show its concerns about possible military links to Iran's nuclear program, a US-based think-tank, the Institute for Science and International Security, said in a commentary.
Iran's envoy to the IAEA, Ali Asghar Soltanieh, dismissed what he called "baseless allegations" about Iran's program.
Nevertheless, he described the report as a step in the right direction, saying it showed that Iran had fully cooperated with the IAEA by allowing a senior nuclear inspector full access to atomic sites during a five-day visit last month.
"This new trend of positive cooperation between Iran and the IAEA should continue," Soltanieh told Reuters.
Western diplomats have dismissed Iran's attempt to show increased openness about its nuclear work, saying it is still failing to address core concerns about its aims.
In addition to addressing the issue of alleged military dimensions to Iran's nuclear program, the UN agency said Tehran had begun installing machines for higher-grade uranium enrichment in an underground bunker near the holy city of Qom.
Shifting enrichment activity to such a subterranean site could offer greater protection against any attacks by Israel or the US, which have both said they do not rule out pre-emptive strikes to stop Iran getting nuclear weapons.
At a separate research and development site, the Vienna-based IAEA said, Iran had started enriching uranium experimentally with a more advanced model of centrifuge than the erratic, 1970s vintage machine it has been using for years.
The report showed that Iran has now produced a total of more than 4.5 tons of low-enriched uranium since the activity began in 2007, an amount which experts say could provide material for at least two bombs if refined much more. Its output of higher-grade refined uranium had also risen.