The U.N. nuclear watchdog's 35-nation Board of Governors passed a resolution on Wednesday calling on Iran to step up cooperation with the watchdog and reverse its recent barring of inspectors despite concerns Tehran would respond with atomic escalation.

The resolution was passed by a vote of 20 in favour and two against with 12 abstentions, diplomats said. It follows up on the last resolution 18 months ago that ordered Iran to comply urgently with a years-long International Atomic Energy Agency investigation into uranium traces found at undeclared sites.

While the number of sites under investigation has been narrowed to two from three, Iran still has yet to give the IAEA satisfactory answers on how the traces got there.

"The need for the Board to hold Iran accountable to its legal obligations is long overdue. Iran must urgently, fully and unambiguously co-operate with the Agency," Britain, France and Germany said in a statement to the Board on the resolution they proposed.

Since the last resolution the list of problems the IAEA faces in Iran has grown, and the new text also called on Iran to address several of those issues.

In September Iran barred many of the IAEA's top enrichment experts on the inspection team, which IAEA chief Rafael Grossi called "disproportionate and unprecedented" and a "very serious blow" to the agency's ability to do its job properly.

"(The Board) calls on Iran to reverse its withdrawal of the designations of several experienced Agency inspectors which is essential to fully allow the Agency to conduct its verification activities in Iran effectively," the resolution said.