The board of governors at the UN nuclear watchdog has passed a resolution critical of Iran, diplomats said Friday, the first of its kind since 2012.
The resolution calls on Tehran to provide inspectors from the International Atomic Energy Agency (IAEA) with access to two sites in Iran in order to clarify whether undeclared nuclear activity took place there in the early 2000s.
Iran has been blocking access to the sites for months.
The resolution was carried by 25 votes in favour versus two against, with seven abstentions.
Russia and China, both of which had spoken out against the prospect of a resolution earlier this week, voted against.
It had been put forward by France, Germany and Britain and supported by the United States, even though the American ambassador the UN in Vienna had said "the text could be strengthened".
Earlier this week Iran warned that such a resolution would be "counterproductive" and that it would take "appropriate measures" in response.
Russia's Ambassador to the UN in Vienna Mikhail Ulyanov echoed that position after the resolution was passed on Friday.
"While stressing the need for Tehran and IAEA to settle this problem without delay, we believe that the resolution can be counterproductive," he tweeted.
Even though the sites in question are not thought to be directly relevant to Iran's current nuclear programme, the agency says it needs to know if activities going back almost two decades have been properly declared and all materials accounted for.
Despite the row over the two sites, the IAEA says it still has the access it needs to inspect Iran's declared nuclear facilities, as per its mandate under the landmark deal between Iran and world powers reached in 2015.
However the latest dispute comes as that deal continues to unravel, with Iran continuing to breach the limits on nuclear activity in the accord in retaliation for the US withdrawal from it and reimposition of sanctions.