Iran's nuclear programme was set to dominate a meeting of the UN atomic agency's board from Thursday after a damning report from the Vienna-based body revealed deep differences between world powers.

The two-day International Atomic Energy Agency (IAEA) gathering was expected to result in a resolution criticising Tehran, but it was unclear how hard-hitting this would be.

Last week, the agency came the closest yet to accusing Iran outright of seeking to develop nuclear weapons, in a report immediately rejected by the Islamic republic as "baseless."

Based on a mass of information from different sources, the IAEA said it was able to build an overall "credible" impression that Tehran "carried out activities relevant to the development of a nuclear explosive device."

The evidence included a bus-sized steel container visible by satellite for explosives testing and weapons design work, including examining how to arm a Shahab-3 missile, capable of reaching Israel, with a nuclear warhead.

Iranian Foreign Minister Ali Akbar Salehi said on Wednesday Tehran would send "an analytical letter with logical and rational responses" to the report.

But the report laid bare deep differences within the so-called P5+1 bloc dealing with the Iran question, the five permanent members of the UN Security Council -- the United States, China, Russia, Britain, France -- and Germany.

Washington, Paris and London jumped on the IAEA report as justification to tighten the screws on Iran, already under four rounds of Security Council sanctions, and additional US and European Union restrictions.

But Beijing, which relies heavily on Iranian oil imports, and Moscow, which also has close commercial ties and built Iran's only nuclear power plant, have been far more cautious.

Both put pressure on IAEA chief Yukiya Amano not to even publish the report, which Moscow said contained nothing new -- even comparing it to the false intelligence used to justify the US-led invasion of Iraq in 2003.

"The Russians and Chinese are in a bad mood, particularly because of what they perceive as a misuse of the last UN Security Council resolution on Libya," Oliver Thraenert of the German Institute for International and Security Affairs said.

Washington and its allies want a resolution that France's envoy to the agency Florence Mangin called "firm, precise and clear, calling on Iran for the final time to live up to its responsibilities."

An IAEA resolution only needs a simple majority to pass, unlike in the Security Council where the "big five" can each cast a veto, as Russia and China did over a resolution condemning Syria's crackdown on protestors in October.

So if the two sides fail to see eye to eye, one option may be an IAEA resolution passed without Russian and Chinese support, in what could be a watershed split.

"I think this would backfire and in the long run not be a good idea," Thraenert said. "It wouldn't help in terms of a new resolution in the Security Council."

Late Wednesday diplomats said that the two sides were nearing a deal, with one Western envoy saying it would call on Iran to intensify dialogue with the IAEA and instruct Amano to give an update in March.

Israel, meanwhile, will be watching closely, following speculation that it might launch a pre-emptive strike on Iran in an attempt to knock out its arch foe's nuclear facilities.