Iran must respond to a report by the UN atomic agency alleging that it is carrying out secret work on the development of a nuclear weapon, US Secretary of State Hillary Clinton said.
Addressing a summit in Hawaii, Mrs Clinton said: "Iran has a long history of deception and denial regarding its nuclear programme, and in the coming days we expect Iran to answer the serious questions raised by this report.
"The US will continue to consult closely with our allies on the next steps we can take to increase pressure on Iran."
The International Atomic Energy Agency (IAEA) showed satellite images, letters and diagrams to 35 nations in Vienna as it sought to underpin its case that Iran is apparently working secretly on developing a nuclear weapon.
Iran's chief envoy to the IAEA, however, rejected the presentation as based on material fabricated by the United States and its allies.
The IAEA report strongly suggested that Iran is using the cover of a peaceful nuclear programme to produce atomic weaponry.
Iran is under UN sanctions for refusing to stop uranium enrichment - which can produce both nuclear fuel and fissile warhead material - and other suspected activities that the international community fears could be used to make atomic arms.
It dismisses such allegations and says its activities are meant to be used only for energy or research.