The United States hopes to get the International Atomic Energy Agency (IAEA) board to pass a resolution urging Tehran to reveal the full extent of its nuclear research to inspectors trying to establish whether it is on the road to developing atomic weapons.
"The US' goal is to pass a strong resolution that will support the IAEA's efforts to get to the bottom of Iran's nuclear programme," the US ambassador to the IAEA, Kenneth Brill, told AFP.
The resolution should "stress the urgency of Iran cooperating fully with the IAEA, which it has yet to do," he added.
Washington believes Iran is violating the nuclear Non-Proliferation Treaty and secretly trying to acquire nuclear arms, while France fears it could succeed within a few years.
Britain, which is embroiled in a diplomatic row with Tehran, has warned it to sign an additional protocol to the NPT to allow surprise visits to its nuclear sites and is expected to take a hard line at the meeting.
But Western diplomats said they would stop short of asking the 35-member IAEA board of governors to adopt a resolution declaring Iran in non-compliance with the nuclear treaty.
Such a resolution could send the matter to the UN Security Council, which could in turn result in sanctions on the Islamic republic.
US envoys have in recent days binned a text on non-compliance because it was clear it would be rejected by the non-aligned countries on the board, who want to give Iran more time and could mute any message from the body.
"The tradition at the board is to seek consensus ... They will not get a resolution to refer the matter to the Security Council," one source told AFP.
"They are likely to wait for the findings of the final IAEA report on Iran in November."
The IAEA's August report to the board of governors confirms that inspectors found enriched uranium at centrifuge facilities at Natanz, south of Tehran.
It also suggests that the Kalayeh facility in Tehran, believed to be used to test uranium centrifuges, was sanitised before IAEA inspectors were allowed in to take samples.
While experts deny there is proof of a weapons programme, the findings have heightened suspicions over natural resource-rich Iran's insistence that its only aim is to produce electricity with nuclear power.
"The countries on the board that have nuclear weapons recognise what Iran is doing because it is what they have done," one Western diplomat told AFP.
The report also worries some non-aligned states, notably South Africa which dismantled its military nuclear capabilities after the fall of apartheid.
This has Western states hoping that if they cannot get the IAEA board to agree on a resolution, they could get one passed on a vote, for which a simple majority is needed.
IAEA secretary general Mohamed ElBaradei is said to favour a resolution as long as it does not alienate the Iran authorities and risks them refusing further IAEA inspections.
"He wants to maintain the agency's role in Iran and does not want them to withdraw from the NPT, which would allow them to develop nuclear arms unhindered," a diplomat said.
Instead ElBaradei wants Tehran to implement an additional protocol and clear up questions on the history of its nuclear programme, including which countries have secretly helped it.
His report reveals "several contradictions" in Iran's story and shows that the programme dates back further than Iran has admitted, hinting that is also more advanced, a diplomat told AFP.
"The Iranians are taking us for a ride. An additional protocol is not enough. It will allow future snap inspections, but what really matters is to get an explanation on what has happened so far in Iran," he said.
Iranian Foreign Minister Kamal Kharazi said Saturday Iran might sign the protocol "in the near future" but only if its conditions were met.