Iran is "steadily" producing enriched uranium, the head of the UN nuclear monitoring organization said in an interview published in The Washington Post on Monday.
Yukia Amano, director general of the International Atomic Energy Agency, expressed concern it might be pursuing military goals.
"Iran is somehow producing uranium enriched to 3.5% and 20%. They are producing it steadily, constantly," Amano told the paper.
Iran is at loggerheads with the West over its nuclear programme of uranium enrichment, and the last round of talks between Tehran and the world powers broke down in Istanbul in January.
Western powers led by the United States suspect Iran is masking a weapons drive under the guise of a civilian atomic programme, a charge strongly denied by the Islamic republic.
Iran is currently under four sets of UN sanctions and other unilateral punitive measures imposed by several countries, including the United States and the European Union.
"We receive information from various countries and collect information from our own sources that give us concern over the possible use of nuclear materials for military purposes - in the past and perhaps now," Amano said.
But the IAEA did not have hard evidence implicating Tehran in nuclear bomb making, he said.
"We are not sure if they are hiding something," Amano noted. "We don't have a smoking gun. We have concerns."