The United Nations nuclear agency, the IAEA, has voted to censure Iran over its nuclear programme.
It is demanding that Iran freezes a uranium enrichment project immediately.
The resolution, the first against Iran in nearly four years, was passed with rare Russian and Chinese support by a 25-3 margin. Only Cuba, Venezuela and Malaysia voted against it.
In September, it emerged that as well as its uranium enrichment facility at Natanz, Iran had a second such facility near the town of Qom.
The IAEA has called on Iran to reveal the purpose of the second plant and confirm that it is not building any other undeclared nuclear facilities.
The United States says time is running out for Iran to address key issues and British prime minister Gordon Brown, speaking at the Commonwealth summit in Trinidad and Tobago, says sanctions are the next step if Iran does not respond to a "very clear vote".
Iran says its nuclear programme is for peaceful energy purposes, but the US and other nations say it wants to make nuclear weapons.
'We have effectively reached a dead end'
The resolution came a day after the outgoing head of the IAEA, Mohamed El Baradei, expressed frustration at Iran's refusal to accept an international proposal to end the dispute over its nuclear programme.
The plan envisages Iran's low-enriched uranium being shipped overseas for processing into fuel. This is seen as a way for Iran to get the fuel it wants, while giving guarantees to the West that it will not be used for nuclear weapons.
But Mr El Baradei, addressing IAEA governors in Vienna on Thursday, said: "We have effectively reached a dead end, unless Iran engages fully with us."